I would just like to start by saying hello to you guys as I'm new to the forum, so "hello".

I have an Aastra 57i which is running through my 2820vn using a sipgate account

My question is. Can I make my Aastra phone use the PSTN line that is connected to the router for outgoing calls?

If so, how?

The Aastra phone is connected through the ethernet not the analogue connections.

Thank you for any help!

Nope, it would need to be set up similarly to how the IPPBX would be set up, with the IPPBX being the SIP gateway (although I don't think the IPPBX 2820 can be used for PSTN lines for much other than analogue handsets, yet).
You'd need to have an analogue phone connected to be able to use the PSTN pass-through.

No Problem. Thanks for the info