It is the phone company, testing the line for problems. See this Straight Dope article:
During off-peak hours a central-office computer goes around injecting a small voltage into each line to check transmission quality. The juice is too low to trigger a ring in most phones, but apparently a few are hypersensitive. The whole thing seems to have started when the phone company began replacing its old electromechanical switching equipment with new electronic stuff. Line testing on the old system never caused problems, but obviously the new system has a couple bugs. When the phone company suspended testing, the ghost rings stopped. When it started up again, the rings returned.
This signal in question is generated by a piece of equipment called a reflectometer, which sends a voltage pulse down a communications cable. By timing the pulse returns caused by impedance changes, the reflectometer can tell how far down the line a problem is, as well as the nature of the problem - whether it's a broken line, a failing connector or even a wiretap device. This signal won't interfere with most phones, but certain types have a ring circuit that is oversensitive. Normally, it takes approximately 90 VAC to trigger a ring, but some phones will trigger on less, hence the problem. The solution: try changing the phone.