Wow that is pretty general with such a wide array of things that may be causing it. First thing comes to mind is that your incoming lines sound as though they are call forwarded. Have you recently changed dial tone vendors, or gotten new service from your existing provider? Has there been any programming done recently to your phone system? Another possibility would be ringing generators within your key service unit going bad or, have you added anything to the system latley which may require more rining generators? It could be any number of things. I would check out the easiest, or most obvious causes first.
We all need a little help once in awhile. Tony the Phoneman..