Could any of you share your experience with remote SIP extensions behind another NAT?

Basically my setup whould be like this:

[ Remote SIP extensions]------>(private IP@) NAT Router B---(public IP @B)-------(Internet Cloud)-----------(public IP @A)  NAT Router A---(private IP @A)------(private IP@C)[Freeswitch]

I have the domain registered as "sip.mydomain.com"


let's see, I am trying to config the remote SIP extension as 1018 using port 5080 hitting the public context, but I got the error on the freeswitch console

"user 1018@sip.mydomain.com not found"

Could you guys guide me to the right direction for remote SIP extensions' deploymen?

Your helps are appreciate.
Thanks,

Chris