Let’s be clear, who the originating carrier is, landline, SIP or other, it all still goes to your SIP provider who delivers to your WAVE. The Wave only takes messaging from and to your provider and possibly to the media server for the RTP stream that your provider told the WAVE to use.
When a call is originated and terminated all within the same provider, it is typically called an “on-network” call as they handle the messaging at each end. In some cases, the provider will respond to the IP seen in the registration and not to the contact and connection header as provided.
The WAVE has a debug function where you can record a SIP trace. I suggest you enable, make a call that exposes the problem and then disable and download and post so we can see what is apt to be causing the issue.