VOIP phones connecting to establish system?

I’m really new to this arena so bear with me.
I’m the IT guy at a site. We have a ROLM MOD-10 9751010 phone system in place that is managed off site. We recently established a short, point to point, wireless site to a warehouse about 200 yards away. It works great and good connectivity on wireless G with 54mb. I’m not running heavy traffic to it. 4 computers just connected to the email system. I’m looking to add 3 phones to this site and tie them in to our system so they can get voicemail and simply dial the 4 digit extension to the contact on this side. Is it possible to get hardware to plug into the phone system, (or jacks or into the pad) to convert it to IP to send it across the network, then convert it back to digital to one of our phones on the remote site?
What do I look for? I’ve done some searching but I think I’m looking for the wrong things.

I haven’t heard of anything that does that but I wouldn’t be that familiar with it anyway. I’d just get a full Broadcore Hosted VoIP PBX system and you can just plug the IP phones into the Internet anywhere in the world that you have a good Internet connection including your wireless site.

I don’t know that particular phone system. Is there any spare connectivity that could be used on it? For example, analogue ports, ISDN, etc.