Do MVNO's use the entire network they buy from? In other words would every MVNO that uses T-Mobile have equal use of T-Mobiles towers or do they only use certain parts of the network?
Accepting that maybe I misunderstand your question, the regular customers to the major carriers(Verizon, TMobile, AT&T) are given priority when the network in the used areas are crowded, while MVNO customers will at times be forced to wait or be given slower responses from the carriers. I am a subscriber to one of the MVNO's and based on my fairly low amount of usage, I generally find no trouble with talk, text, or data. The "certain parts of the network" part of your question I do not understand.