2" is minimum,(800.52(A)2) however more is wise, especially parralel.
But sure it is "static", and not "hum"?
Static can be many reasons. Loose / dirty connection, bad equipment, a whole range of things.
Hum could be induced in the way you described. Especially in PVC or cable. Metallic conduit would afford some shielding effect. Or, the use of shielded phone cable. Both would reduce it to some degree.
Isolate the noise by removing elements of the phone and gate wiring, or shutting off the gate to see if it goes away. If there is an intercom to the gate via phone that could be an issue too.
Wish you luck...
"Well - I oughta....." -Jackie Gleason