OK I've been doing extensive testing today (I'm surrounded with phones!). I've found a very simple way to reproduce the problem. I've also confirmed that the problem does NOT happen with PBXnSIP v188.8.131.52.
You can reproduce the problem without using an external trunk like this:
Hunt group 300 -> rings 203, 204, 205
1) Make a call from 200 to 201. Leave it on speaker-phone.
2) On extention 201, put the call on hold.
3) Extention 200 should be playing hold music out of the loudspeaker.
4) On extention 202, dial 300 (the hunt group). This should ring 203, 204 and 205.
5) As they start ringing, the music playing out of 200 will break up for a second or so. There's also some break-up when they stop ringing. The more extentions in the hunt group, the worse the break up will be. If you're just ringing one extention there's only a small amount of break up.
I've tested this on Windows Server 2003, and Windows XP - they all have the same result. All v2.x versions give the same result. The handsets are Snom 360. I've tested it on Netgear and DLink switches.
Most importantly, when using v184.108.40.206, there is no break up at all. This pretty much proves it's nothing to do with our network here, and it must be a bug in PBXnSIP.
Can you reproduce this behavior? Is there a work-around? I really don't want to downgrade to v1.5, because we now archive all our calls (an excellent new v2.x feature).