Voice part of call dropping at 30 seconds

[Ambient]

Hi Everyone.

Whilst my D-Link all in one voip router is in for repairs, I decided to bring out my old modem and my Netcomm V100 ata adapter (which, did work fine before). After configuring and the like I make a call and what I'm noticing is that around 30 seconds into the call the voice part just turns off. The call remains connected and I can see data coming down but, no voice.

Strange. So I've tried my 3 different SIP providers directly and there is no problem at all. Go back to using Voxalot and sure enough, every time it'll drop at around 30 seconds.

I'm forwarding ports 5060 and 6000 for both TCP and UDP (the V100 has an RTP baseport option, so I just forwarded it on my NAT anyway) and apart from that there's not a lot of options I can change to diagnose this issue further.

I've tried all combinations with Optimise Audio Path and Symmetric NAT handling on the Voxalot side to no avail as well. Can anyone suggest anything else to try?

Thanks.