I've heard of that happening although we can't really do much to help you unless you give us the model of the routers as well as the splitter. It'd also help if you told us what type of routers you were using (cable modem/dsl router).
Not knowing what your routers are I would guess that you're using a store bought splitter designed to let your phone cord get split into two and that you're using dsl. Whenever the phone rings, it's connection interferes with your dsl connection. There are special filters (that should have come with your router) that prevent this from happening as well as splitters (which should have come with your router) that do a much better job of filtering the correct frequencies and stopping them from interfering with each other.
