I'm not sure if Grady would be charged "rent" to use the radio space. Grady does operate in Fulton County but only in the City of Atlanta. Grady does provide mutual aid assistance to RMA in North and South Fulton.
Rural/Metro Ambulance Service operates in North Fulton (City Of Milton, City of Sandy Springs and City of John's Creek) (possibly Milton County soon?) and South Fulton (City of East Point, College Park, Union City, Fairburn, Palmetto, City of Chatt.Hills, etc. RMA uses Fulton's radio system just like Grady does now.
I guess it would make sense that Grady would operate on COA radio system since they respond to calls with them.
I can't imagine why someone else would want to severe ties with Fulton County? Can you?