I've not done that personally, but I'd have a look on the freeswitch forums to see if someone on there has already done it.
I have had the free 3CX sip client on an android mobile, using a UDP vpn back to a freeswitch server which was connected to the UK phone system. Proof of concept thing.
Because of the nature of the mobile data, which is like bursts of data, it made it hard to have a conversation even around midnight and 1am when cell traffic volumes are low.
I was told UK Telco's give voice data priority as this needs to be realtime and then other data has lower levels of priority making it hard to have a reliable stream of data.
FYI, 56K is the minimum standard for voice calls, although with 4G and beyond, its possible to hear the higher bitrate protocols being used on the main mobile networks, as its not so muffled, like someone turning up the high end frequency's on a graphic equaliser, and they are fully down with the 56k protocols.
I dont know what the minimum video protocol bitrate would be, but I did find this which might be useful.
> H.264 will only consume around 10 kbps with around 2 fps in 176x144.
When Skype first came out, it was peer to peer, so as a proof of concept to test the new 3G network I did manage to maintain a voice call for an hour which was impressive, but also shows that mobile networks can maintain the mobile data streams if its not encrypted inside a VPN in the UK.