here at LGA
we have a combination. depending on what part of the ramp the a/c is parked on, they will first call our airline-operated ramp control but will get pushback instrucions from either ramp or ground control. 6 of our gate pushback onto an active taxiway so the need for ground is important. problems arise when wx delays or other such situations occur. we can incur 20-30 minute delays on pushes.
the other three ramp control towers are operated by AA, UA and DL. we (US) run the ramp around our terminal. each tower has control over what they can see, so if you are parking on a gate that cannot be seen from, say, the UA tower you may be handed over to the AA or even our tower for parking cleanrance. it all depends on what the towers can see. now... i need sleep.
[Edited 2005-06-12 07:41:38]