Quite a bit of Technical Information on SIP2SIM!

SIP2SIM Technical Information

There are three main ways to set things up:

  • Using with A&A service and number, calls/texts work like a normal mobile service, but can work on non 07 numbers , such as 01, 02, etc (not all non 07 numbers handle texts).
  • Using with your own SIP based phone system like a SIP VoIP extension.
  • Using with a separate VoIP provider.

Using with an A&A number

You can request the SIM/eSIM is associated with a number on A&A VoIP service. This does not have to be an 07 mobile number, it can be a landline (though SMS to landline may be limited).

Leaving details blank (the default) on the SIM settings and Number settings will tie calls and SMS each way to the number. It is possible to add additional settings to the number to have calls to your server and the SIM/mobile at the same time. You can also set incoming SMS to be to other targets (email, http, mastodon, mobile numbers, etc) and to one or more ICCIDs for SIM/eSIM service.

All of our normal services work, including call recording.

Using mobile as if it is a SIP/VoIP handset on a phone system

The SIP2SIM service registers with a SIP endpoint using a specified hostname, username, and password as set on the control pages. This registration comes from one of our SIP service IP addresses (see more here). For maintenance, the IP used can change from time to time, and will usually mean a de-registration is sent from the previous IP and a new registration from the new IP.

Calls made to the registered contact are passed to the SIM/phone. This works with most SIP based phone systems and most SIP carriers.

Calls from the mobile are sent to the hostname specified, using the username and password specified, if specified. The called number is the dialled number from the phone. Unless we have directly associated one of our numbers with the SIM, the CLI is the SIM's ICCID. The call comes from the same group of call servers.

Using with another SIP/VoIP provider

You can specify their hostname, username, and password, just as connecting to your own SIP server. This will work with many providers, but check their terms as it means telling us your credentials, which may be against their terms. This also can limit our ability to resolve issues if their response is that it is against their terms.

Technical Tips

  • Calling withheld from the phone is usually a problem, we may have a fix for this in due course, but for now, ensure you are not. For A&A services, prefix your call 141 to withhold. You can enable sending CLI by dialling #30#.
  • If you are seeing issues with calls to the phone getting a 488 error, make sure VoLTE is turned on.
  • If you have issues with Data, lock it to 4G only for now.
  • If you want to move an eSIM to a new phone, you need to do a DELETE on the old phone, and ask support for a new QR code to install on the new phone.
  • If you are using a landline number, SMS to you may well be a call reading it out, but we are working on that.
  • Yes, an SMS from Amazon or Barclays appear as from +Amazon and +Barclays, and so on, it is being worked on - it is not us, honest.
  • Ensure your phone s/w is up to date, especially if not yet on Android 13 or newer.

Technical details

  • In the UK the service uses EE with a fallback to O2.
  • The service is best on 4G/5G with VoLTE enabled, but with a 2G fallback possible.
  • Annoyingly, VoLTE is not as simple as old school 2G, and so it is important to ensure your phone software is up to date. Even quite recent older Android versions may not work well.
  • All calls each way are UDP SIP/2.0 control messages with G.711 64kbs A-law audio RTP. The service uses DNS and SRV records.
  • Some mobile phones validate dialled numbers and may not allow dialling of short sequences, such as three digits.
  • The dialled number on a mobile phone is normally presented as dialled, but if dialled with a + prefix, the plus is usually lost and only the digits are presented. This means you need to dial 00 prefix instead of + when the SIP server needs to see international dialling codes.
  • Calls to 999 or 112 will go direct to an emergency services operator. Some phones hijack other numbers such as 911. At present the network hijacks a few other short number sequences. It is best to test if an extension number works with the service.
  • Calls starting * and #, and followed by just digits, are passed to you, but some combinations of * and # can be used for normal GSM settings and are processed by the network.
  • If you tell the phone to withhold your calling number, calls from the phone won't work. SIP2SIM logically has no number anyway, so you need another way to present a withheld number. Our VoIP services uses 141 as a prefix for this.
  • Changes to the registered SIP server can take up to a minute to take effect.
  • If using an A&A VoIP number, and it has not been pre-assigned to the SIM, set the hostname to voiceless.aa.net.uk and use the number and password assigned just like registering to our service from any other device.
  • Some phones (for example some iPhones) may prohibit data tethering - however we are working on this.
  • The service is mainly intended for using with A&A numbers or with a SIP based phone system. It can work with other VoIP providers. Please be aware that using with another provider means sharing your login details with us, which may be against their terms.
  • Some VoIP servers such as asterisk are stupid if we de-register with one call server and register from another and there is an overlap. The overlap should ensure no gap and that calls work, but instead asterisk takes the latter de-register operation as cancelling the first even though a different contact. This is a fault in asterisk and you should complain to the developers. Thankfully we do not upgrade the servers that often and so do not need to move registrations, and the problem fixes itself on next registration (typically 30 minutes).
  • Some VoIP providers do not support compact SIP. This is not RFC compliant (RFC 3261 says "Implementations MUST accept both the long and short forms of each header name.") so SIP2SIM will not work with these providers until they fix their service.
  • Some VoIP providers do not send calls to the Contact we REGISTER. This is not RFC compliant so won't work until they fix their service.
  • SIP2SIM is known not to work correctly with Sipgate where multiple Sipgate registrations are involved in a call. We cannot work with them to fix it as they say using SIP2SIM is against their terms, sorry.
  • Some phones have been quirky with eSIMs. Please bear with us if you have issues.
  • If you want to a transfer an eSIM to a new phone, use the old phone to Delete SIM first, and ask us for a new QR code.
  • Outgoing MMS probably won't work - who uses that?!
  • WiFi calling does not work at present, maybe one day.
  • Apple Watch secondary SIM working, not yet, sorry.


The text service allows texts to and from the phone with the SIM. Texts can be sent to the phone by using our normal text gateway with a destination of the SIM ICCID and credentials for the text service (e.g. a VoIP account). The control pages allow texts from the phone to be sent to an HTTP address of your choice.

Some phones and some networks validate the number you are sending a text from and will only reliably allow texting of a full phone number.

If you have an A&A VoIP number on the same login as the SIM you can set the SIM ICCID on the incoming section of the VoIP number so that texts to that number are relayed directly to the SIM. There is a text charge for this. Not all of our VoIP numbers can accept incoming texts.

If you have an A&A VoIP number on the same login as the SIM you can set this on the SIM config on the control pages so that texts from the SIM are sent out as from that number. This incurs our normal outgoing text cost plus the SIP2SIM text cost, but we hope in future to apply a discount in this case.

There may be a small range of non UK mobile numbers to which mobile originated texts would go directly, i.e. bypassing the normal http posting, and presenting a different sending number.

At present there may be ways for some texts to reach the phone bypassing the http posting (i.e. a hidden number) but we expect to be able to fix this in due course.

Some phones (e.g. iPhones) send SMS to validate services (e.g. iMessage) and these will be chargeable.

Not all A&A VoIP numbers can handle incoming texts. There is unlikely to ever be any discounting of inbound text charges even when we have 07 mobile numbers working for texts as we are unlikely to see incoming revenue from such texts.

Note: At present SMS to mobiles starting from a non number are prefixed +, e.g. +Barclays, etc. We hope this should be fixed soon.

Mobile data

The SIM is pre-set with a monthly cap as part of the price. We hope to have more details soon.

Please do not expect any better than NAT IPv4 data. We hope one day to do better.


Roaming is no extra charges in EU and US. No details on other roaming yet.

Fair usage limits

  • 10,000 min/month calls.
  • 5,000 SMS/month, no spam or fraud please..

SIM Application menu

The SIM has an application menu for IMSI. This normally should be set to automatic to allow Roaming.

CLI shown on mobile

The CLI for calls to a mobile should be sent as a full international number without the leading + and should be one of your numbers (or the calling number if passing on a call you have received). Other formats may work (e.g. nation number starting 0).

However, due to OFCOM rules to stop junk calls (which clearly did not help and just caused problems we now see here) sending a call which does not look valid, e.g. an extension number like 1234, does not work and the call may fail.

To work around this we check for a CLI looking sensible and if not we replace it with a new CLI - we include up to 9 digits from the end of the original CLI, so for example, 1234, will present as +11000001234, which shows on an iPhone as +1(1) (000) 001-234. This is to allow you to see the original CLI if needed. +11 are not valid numbers so this will not be a real CLI.

Unfortunately it also seems that a CLI that matches the number we have assigned to the SIM always reports as Busy even when the handset is not busy, so this two gets replaced in the same way.


Apple phones work with iMessage, and they find their own phone number by sending an SMS. This is handled internally within the network, and means that if we have associated a number with the phone when you ordered the eSIM/SIM, it will work for iMessage. It has to be one of our numbers and live on our network and on the same login on our control pages. This means you do not see the activation SMS sent to your SMS server.

This even works when it is a landline number that would not normally be able to get SMS (we are working on this anyway).

Other messaging services typically work by sending an SMS with a code that you enter, so this will work as long as you get the SMS (which could be to the phone, or email, or even called as a voice call if a landline number), and so typically work without problems.

OFCOM have a number of rules, and one is that most 07 numbers must only be used for mobile (i.e. where the call has a radio link to a mobile device). Importantly there is no requirement that mobile services have to use 07 numbers, so mobile services on landline numbers are quite valid. There is also nothing to stop a call to an 07 number calling a mobile and a desk phone at the same time, or going via your phone system that then goes on to our SIP2SIM mobile service, or forwarding to another 07 mobile service. All mobile operators break these rules all the time with voicemail and call diverts with no action from OFCOM, but we aim to comply as well as we can. As such we ask that customers of 07 numbers try to ensure they are used with mobile devices, at least most of the time.

Contact Sales

email sales@aa.net.uk
phone 03333 400222
(Mon-Fri 9am-5pm)
sms 01344 400222

Contact Support

email support@aa.net.uk
phone 03333 400999
(Mon-Fri 8am-6pm,
Sat 10am-2pm)
sms 01344 400999