What office phone system do I have and how do I update it?

My two-employee (with an occasional intern) office has a very old phone system, one with probably more features than we need. We have four lines coming in. A few weeks ago, something strange happened to the phones, making them not work the same way they used to. Since this system is so old, we do not have any of the telephone or system manuals and don't know how to fix it. We may need new phones or an entire new system.

The current phone seems to be a PBX system. The cabinet and three phones are all Fujitsu. I guess my question is can I just buy new 4-line phones (nice and cheap) and us them with the current cabinet, or do I also need a new cabinet (not so cheap)?

Our office would probably be fine with a key-less system, just using the phones, but I imagine that would involve a removal of the current cabinet, which would also require professional installation (un-installation) services. Any ideas are appreciated. We are looking for the most cost-effective option. Thanks.