I am relatively new to VoIP so please forgive me for the basic question.

I currently use VoIP for my office. I have an IP phone and the accounts are registered on there (Grandstream GXP2020) and I connect to my providers SIP server. In essence, my 2820Vn is just used as a a Gateway to the Internet and the internal VoIP features are not utilised.

However, the 2820Vn can handle the SIP connections internally so I thought I might use that instead of the phone device. (Well, at least plug another device into the Draytek it and test the VoIP features out!) And that's when I come across the issue. How do you plug an IP phone with its Ethernet Cable into the 2820Vn and configure it to work? Or is it only meant to work with a normal phone?

Do you still need to use the phone ports/voip facility of the router ?

I was intending to use the VoIP facilty of the router but plug in a normal IP phone such as Grandstream GXP2020 via the RJ45.

Is that possible?

Yes. You'll need midspan injectors or local PSUs though. Then either a SIP logon somewhere or config the Draytel service.