asterisk users - Dec 2003 - grand stream phone and double nat

Hi

I'm trying to configur a grandstream BT101 to connect to asterisk, both 
behind different NATs, I realise that a double Nat is a problem, I have 
tried using fwd  forwarding to iaxtel as a solution but cannt seem to 
get them to connect as I think there is a codec problem as IAXTEL 
doesn't seem to accept alawor ulaw is this correct?
Has anyone been able to connect a sip phone across a double NAT ?

I realise this has been discussed before and sorry for that

Robb