As per title, I have an odd issue with my VOIP setup. I have a handful of Android smartphones with Zoiper that have been connecting to my local FreePBX box and generally work great with minimal issues. However, I've noticed that if I do an attended transfer an incoming call from one handset to another, I'll get a lot of packet loss and the call quality slowly degrades significantly to the point where it's very difficult to have a conversation. The two devices are not even using the same Wi-Fi access point (which are both connected via a gigabit connection) and there isn't any congestion on my local network. All other calls work great so it's nothing to do with my provider.
If anyone had any ideas that would be great.