We are an elementary school

We are an elementary school with two campuses located 4 miles apart. Our phone system and our computer system are both intertwined. We can call the other building using an extension. Everything worked fine for a few years -- but in the past year, we have had numerous problems. During a call, voices sound garbled and often only every other word can be heard. This does not occur when making outside calls. The server is at our south campus; the north campus experiences VERY slow computer response. Help!