I was wondering if anyone was aware of reported issues with the classic 'one-way' audio problem when using a DrayTek 2820Vn router. We have 3cx hosted in a datacentre with relevant ports open 5060,5090,9000-9049 and two-way external calls are successfully made from another site (using GXP2000s again). Another remote site has a Grandstream GXP2000 handset (DHCP) behind a single nat DrayTek 2820Vn (not using the voip features of the router in this case). EDIT - in both cases the phones report NAT type is full cone and are using stun.3cx.com The server log doesn't indicate that RTP cannot be delivered so the call is expected to work ok. I have tried setting a 'sip_alg' setting on the DrayTek using telnet, but this had no effect. Can anyone suggest what the problem is, or a way to get some more information so I can work out what's going on?