It's interesting how this subject keeps coming up after five years or more. I have been using this direct ip calling method reliably using wifi and 4G for several years:
http://www.obitalk.com/forum/index.php?topic=8511.0I keep an OBi110 running partly to carry out this function. I don't have an OBi2xx series to try it on. It definitely does not work on an OBi1032 IP Phone. The keep-alive STUN method does not seem to have been implemented even though the setting are there - that's a very low priority for Obihai firmware writers I would guess!
Warning: There are some variables with this setup, so YMMV. The main problem is likely to be routers seeing this unregistered voip traffic as unsolicited and blocking it. If this is a problem for you, then use one of the first two methods in azrobert's post which use SIP2SIP as a free registered service to connect your cell / mobile phone to your OBi.
What we are really talking about here is trying to find a work around for the fact that OBiON now works badly or not at all on most mobile platforms. The best answer for us OBi users would be for Obihai to provide a gateway to their servers using standard SIP protocol rather than their proprietary OBiTALK protocol. Then we could use any suitable softphone such as Zoiper and use our OBi log in details to access the Obihai servers as a gateway to our other OBi endpoints. This would mean Obihai could officially give up on OBiON and leave other companies the problem of continually updating their softphones.
Sometimes I think that Obihai just like setting us challenges