We have an IP phone system in our office, and about 8 VoIP phones running on the system. We have a remote worker, who is literally states away.

We'd like to connect his phone to our VoIP network, so that he has a business phone and an extension to which we could transfer calls.

I was thinking, although I don't know for sure, that a pair of Cisco routers could be used in some way to make this work. I imagine a VPN solution, where I have one router connected to the phone network and the other router connected to the remote phone. Then have a site-to-site VPN set up so that the remote router...

And that's where I'm stuck. I know the remote router will need to use the DHCP server of the phone system. I've never set up something like this, so I am seeking the help of the community here.

What is the best way to get this remote VoIP phone RELIABLY connected to our internal VoIP network?