Make sure none of the routers are using SIP ALG including the ones you are connecting through to the VPN. SIP ALG prevents detection of NAT which can cause issues with the SIP stack placing the proper IPs where needed. With out full details you may also need an SBC license for the Adtran so you can use RTP Media Anchoring to force the 924E to stay in the middle of the call. Without Media Anchoring the 924 will attempt to directly connect the two RTP stream endpoints and since the remote side doesn't know how to route to the upstream provider the RTP breaks. With it the remote phone connects to the Adtran 924 and it creates a new separate call leg to the far end and it then connects the two separate call legs.