Replacing A&A mobile service
So, I have been looking for a replacement. It is possible to use a data SIM in the iPhone, but with our data SIMs the texting does not work, so only iMessage. We use texting a lot for nagios, so I had to get some simple contract and there are some that allow lots (unlimited even!) data.
So, can you use SIP on an iPhone over 3G?
So far the answer is yes! There are a lot of SIP clients that seem tied to specific SIP providers. But this one "3CX" was the first when I searched for SIP and rather shockingly seems to just work.
I have turned off STUN, NAT helper, and echo cancellation, forced a-law audio (probably unnecessary), and registered to my FireBrick. The NAT helper and STUN has to be off so that the FireBrick sees it is NAT and does the needful, but then it does work.
The result is a phone that, well, works. The audio quality is perfect (only tested from my house where I have good coverage). Ironically there was some initial break up when using WiFi but on 3G it was faultless. The NAT keep alives work. The app switches WiFi and 3G cleanly, re-registering. Not tried taking a call on WiFi out of range, that would be a challenge I expect, though I may be able to do some sort of call park I expect. I can hold and call transfer cleanly. It means I can record calls (though the app offers that as well!).
I can arrange for incoming texts to relay to the phones "real" number, leaving me only with outgoing texts not using my normal number. Apparently if I had an android then there is an app for that too :-)
Of course, the nice thing is, I can set incoming calls to try SIP and fall back to calling the mobile number. That way I get the best of both worlds.
I'll see how it goes, but so far I am impressed.
Update: Some maths. If we used a data SIM which we charge 2p/MB+VAT for, an a-law call is 160 bytes a-law plus 12 bytes RTP plus 8 bytes UDP plus 20 bytes IP, so 200 bytes each way per 20ms, so 20KB/sec usage. That works out 1.2MB/min which is 2.4p/min for the mobile leg. That is not far off what we did before. Shame data SIMs can't handle text else this would be a very close solution.