I just got a 2710VDn and trying to set up SIP. It works OK for outgoing VOIP calls, but for incoming VOIP calls I always get a one way audio connection. I can hear the remote caller, but they can not hear me. It's the same problem no matter if I answer the call on a normal phone connected to the FXS port or if I answer on a DECT phone registered to the 2710VDn.

I have tried different firmware (V3.3.6+AnnexA_232201, 211011, 243601), but get the same problem with all of them.

Did anyone else manage to get SIP working properly on the 2710VDn? Does anyone have any advice?

One way audio is usually a NAT problem associated with using an ATA with a separate router. The 2710V shouldn't really have a problem as its an integrated ATA/Router.
Seeing as it seems to be OK with outgoing calls my only suggestion is to perhaps experiment with different default codecs, maybe there is a problem with codec negotiation with incoming calls ( although you would have expected that to affect audio in both directions).

I had all kinds of issues when I first set up my 2710Vn. I experienced the same thing at first with my Draytel account and I ended up having to configure it with the nat address.

A few months down the line - 'something' changed and it all started working without nat. Go figure.