Telephone service between upper and lower Montgomery County was interrupted for hours yesterday when the principal cable linking the two areas was cut accidentally by a private contractor using a backhoe.

The accident occurred in the 5500 block of Randolph Road about 9:30 a.m., and north-south service was not fully restored until about 9 p.m.

The cutting of the cable, which can handle 21,600 calls simultaneously, made it difficult or impossible for calls to be made between Rockville, Gaithersburg and other areas north of Randolph Road on the one hand, and the District of Columbia, and county areas such as Bethesda that are south of Randolph on the other.

Persons trying to make such calls frequently got only busy signals, a telephone company spokesman said. He said calls within areas to the north and to the wouth of the break were not affected.

Before all 900 individual pairs of wires in the four-inch cable were reapired, some calls were routed around the break by use of toll call facilities, the spokesman said.

He said only about 100 customers in the immediate area of the break were completely cut off. Their service might not be restored until early today, he said.