Quite a bit of Technical Information on SIP2SIM!

SIP2SIM Technical Information

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 ( or 2001:8b0:0:30::5060:0/112). 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.

Calls from the mobile are sent to the hostname specified, using the username and password specified. The called number is the dialled number from the phone. The CLI is the SIM's ICCID. The call comes from the same group of call servers and will normally be from the same IP address from which the registration request is sent.

All calls each way are UDP SIP/2.0 control messages with G.711 64kbs A-law audio. The service uses DNS and SRV records.

  • 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, but are likely to present with a + and the digits sent and may be replaced with a fixed number. We had hoped to improve this but OFCOM rules on CLI passing (unsuccessfully) aimed at addressing scam and spam calls have limited our options on this.
  • Some networks present an in-band audio recording before clearing for busy or no reply, etc. We are hoping to be able to remove this in the future and just send the normal clearing code.
  • 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 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 three digit numbers such as 901.
  • Calls starting * and #, and followed by digits, are passed to you, but some combinations of * and # can be used for normal GSM settings and are processed by the network. On dual identity SIMs, you may also see the phone reporting that it is sending some codes, such as *02# - these are to do with profile switching and nothing to be concerned about.
  • 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.

  • At present there may be ways for some calls to reach the phone bypassing the VoIP (i.e. a hidden number) but we expect to be able to fix this in due course.
  • If using an A&A VoIP number, set the hostname to voiceless.aa.net.uk and use the number and password assigned.
  • Some phones (for example some iPhones) prohibit tethering - this is not something we are doing, and we hope eventually to be able to find a way to fix this.
  • 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. This is a fault in asterisk and you should complain to the developers. Thankful we do not upgrade the servers that often and so do not need to move registrations.
  • 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.
  • SIP2SIM is known not to work correctly with Sipgate where multiple Sipgate registrations are involved. We cannot work with them to fix it as they say using SIP2SIM is against their terms.

Some numbers have special conditions, specifically 07 mobile numbers. These must be sent over radio to a mobile handset. OFCOM have agreed that sending incoming calls to a SIP endpoint that we know is connected via radio to a mobile handset is allowed. This means SIP2SIM qualifies, and so it is possible to set an 07 mobile number to a SIP2SIM connection and accept incoming calls.


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.


To use data, set the APN to mobiledata with no username or password.

At present the data makes use of a NAT Internet connection which is unfiltered. We hope to have a more direct APN in due course.

Some phones will need the APN setting, and may need it setting for each profile separately on O2/EU SIMs.

SIM application

Some types of SIM have an application which can be accessed on most phones as a SIM settings menu of some sort. This allows you to switch the profile between O2 UK (only works on O2 in the UK), a Europe roaming (works in EU, including roaming in the UK to other operators), or world roaming (works anywhere). This can also change automatically, e.g. if set to UK roaming and O2 UK is found as a network.

Different types of SIM

There are different types of SIM available, the full roaming version with the SIM application, and SIMs that are only O2 UK with no roaming. The latter are intended for use where only O2 UK usage is acceptable and for any devices that do not work well with a SIM application for any reason. We may also have some that are just Vodafone NL for telemetry usage (expensive data, but works on any network).

SIM cut out SIM Sizes

Sim Cutout

The SIMs are provided on credit card sized (full size SIM) with push out for mini and micro SIMs. The data SIMs have a nano SIM push out too, but the voice SIMs have a printed outline for a nano SIM so you can cut it down. We recommend using a proper cutting tool, but scissors usually work. Cutting down SIMs is at your own risk (for damaging the SIM or your phone).


Our voice and data SIM cards are ideal for a corporate network or for a wholesale customer to set up their own mobile service.

We can offer branding with professional quality SIM printing and pre-set settings.

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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
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Sat 10am-2pm)
sms 01344 400999