Air Carrier departing SFO being handled by company Ramp Control was questioned by Ground Control regarding ATC communications contact point.

Narrative

Post engine start up; we advised Company Ramp Control that we were ready to taxi. Ramp instructed us to 'taxi to Spot 10 and contact Ground Control. We proceeded as instructed and began to monitor Ground Control frequency. Ground Control suddenly advised us to stop! The Ground Controller further advised that in the future; we initiate a call well before reaching the spot. Identification; SFO Ground Control initiated a call to us; instructing us to stop. The event occurred because we complied with Ramp Tower instructions. The pilot not flying was trying to initiate radio transmission; however could not get a break. Intersecting parallel taxiways were clear and aircraft were holding at alley way spots; therefore I was not concerned about any possible incursion. We as a flight crew were under the impression that ramp would issue such an instruction per Memorandum Of Understanding (MOU) with SFO ATC. We stopped as instructed; and completed our reposition taxi without further incident. In the future; I will ensure that a call to ATC is made before nearing a spot that lies in the area of an airport taxiway. Airport Chart suggests that we; the flight crew and Company Ramp were operating in accordance with protocol. Perhaps the assumption may be made that a there was a breakdown in communication between Company Ramp and SFO FAA ATC.