Hello
1st try to use ulaw, instead of g729.
Second- g729 is codec not prorocol, and if you talking about "IAX" -it's Asterisk to Asterisk connection protocol, and you can use them.
Last one- using of OpenVPN is not good practice for voice communication, better use IPsec, and, i dont understand, what you mean under "blocks in 2 min"- do you have some govermental restruction? Anyway. the is at least two solution, chat to me, this is trivial, dont need to pay for it