It would be impossible to say what the problem is from the information you give.
I doubt the fact that the cable is not of the direct burial type would be an issue at this point- I'm assuming that it only has been there a short time. It will fail eventually to be sure.
We need to know if this is static as you say or hum. Static would not be caused by proximity to the 120 volt cable, hum would be a possibility.
What is the phone setup in the house- stand alone phones or a system? How many lines and do all lines have the static? How was the gate phone connected to the phones or system?
-Hal
