I have a Gigaset C460IP and it's working just fine over Plusnet's network.
It would be useful to know which Gigaset model you have, what type of router you're using, and the speed of your broadband connection. Also, do you have any problems with other traffic being lost on your broadband link?
The fact that you can set up and clear calls OK, both incoming and outgoing, and also establish a voice channel, suggests there shouldn't be any problem with port configuration. For what it's worth, my Gigaset is configured to use port 5060 for SIP signalling (that's the standard SIP port), port 5004 for RTP (that's the voice connection), and NOT to use random ports. There's not a lot else I can set up on the C460IP. I have no special configuration on my router.
With VoIP the voice is carried by RTP over UDP. So there's no guarantee of end-to-end delivery, but RTP can detect lost packets and out-of-sequence packets. The voice codec usually just inserts silence or low level white noise to cover lost packets. The question is where the packets are being lost, obviously.
If you're running Wireshark at your end and can capture the RTP stream, it will show you if there are lots of "missing" packets on the incoming stream (as well as out-of-sequence, delay, jitter, etc), but that doesn't help identify where
they were lost! Also, I assume you're monitoring on the VoIP client side (LAN side) of the router, so you're just capturing visually what your ear is already telling you. I guess you should also be able to see that Plusnet is assigning the correct priority class to the RTP packets.
In my experience voice break up is usually caused by having insufficient capacity on the access link (ie your broadband circuit) because some other traffic is hogging the bandwidth. Assuming Plusnet is assigning the correct priority class to your RTP packets, that shouldn't be happening.
So the only other thing I'm wondering is whether your router is trying to do something with the VoIP packets as they go through it and isn't keeping up. Are you using a Technicolour router? I think they implement a SIP Application Level Gateway in some models, which you can turn off with the telnet commands:
connection unbind application=SIP port=5060
But if that was causing the problem, I'd expect you to have difficulty establishing VoIP calls as well.
If you have another router available, you could try that to see if it fixes the problem. If not, then the problem is obviously upstream of you.
And finally, one other possibility: if it's a cordless Gigaset handset then it uses DECT for communication with its base station. Is that sitting next to your router? It could be that the router's wifi is interfering with the Gigaset's DECT transmissions and you're losing data on the air interface. Try turning off the roter's wifi and see if that makes a difference.
Sorry this is rather long and rambling - just thinking through your problem out loud. Hope something here might help.
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