Android-iOS VOIP Voice Call App

We are currently working on our mobile application, GCM Dialer, which handles both SIM and SIP calls. We have successfully implemented the conference screen for the SIM and SIP merge option, but we are encountering an issue where voice is not being exchanged between all parties.

Below is the scenario we are testing:

- User-1 has the GCM Dialer app installed and set as the default dialer.
- User-2 calls User-1 via SIM, and they can exchange voice successfully.
- User-3 then calls User-1 via SIP. User-1 answers, which puts User-2 on hold. User-3 and User-1 can exchange voice successfully.
- User-1 clicks the "Merge" button, and all three parties are brought into one call screen.

The issue is that while User-1 can hear and speak to both User-3 and User-2, User-2 and User-3 cannot hear each other. User-2 can only hear User-1, and User-3 can only hear User-1. Essentially, User-1 can exchange voice with both parties, but the two external parties cannot exchange voice between themselves.

Could you please review this scenario and provide guidance on how to resolve this audio routing issue during the merge?