I don't know about voxalot, so this reply is probably pointless. But...
I sometimes use the virbiage soft phone (firefly) with Maxotel, using only an ilbc codec. Works great over low bandwidth connections (tested using 3g with an old blue bigpond usb nextg turbo modem, bp3-usb, in Taiwan with a Chunghwa 3g sim card). Bit of lag (for many reasons) but functional.
Not functional enough for everyday business use, but perfectly fine for home ph calls, friends, etc.
My guess is, that if you configure the voxalot service to proxy the sip traffic then voxalot MUST support the codec.
If you configure voxalot only as a 'sip tcp traffic manager' then maybe they don't need to support the codec. (set symmetric nat handling to NO, set optimize audio path to YES). Hopefully someone from Voxalot can give a better answer.
Right now, the brisbane server is down, so if your testing it, check on ozvoipstatus.com before giving up.
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