Softphone issues over Site to Site VPN

[nwcomputing]

Hi Everyone,

I have a site to site VPN setup from onsite to a virtual 3cx instance hosted in AWS. All works fine apart from I am unable to take someone off hold because the ACK packet from the softphone isn't hitting the pabx from the softphone.
Voice sounds real clear, everything else seems to work fine, but from the captures it is clear the ACK packet isn't hitting the pabx when I try take them off hold.

When I register over the internet rather than the site to site VPN it works fine.
Any ideas what I need to change with the site to site VPN? Anyone seen this one before?

 

[NickD_3CX]

If I had to guess, in the "Contact" header of either the INVITE and/or the 200 OK from/to the softphone, the IP will be incorrect.
There can be a few reasons why this could be happening spanning from incorrect 3CX installation to the VPN setup, but as a starting point I would run a packet capture on the PC using the 3CX Client and checking the SIP from there.
If you are right about the ACK not arriving, in the capture you should see to which IP the client is sending it and from that it may make it more obvious where the problem is.