Boost and Virgin are owned by Sprint, which run on the Sprint CDMA network. There's no GSM network for any of them. Verizon is the other carrier with a separate CDMA network. T-Mobile and AT&T each have their own GSM networks.
ETD means that the Eustachian tube is blocked or does not open properly. Air cannot then get into the middle ear. Therefore, the air pressure on the outer side of the eardrum becomes greater than the air pressure in the middle ear. This pushes the eardrum inward.
The primary function of the Eustachian tube is to ventilate the middle ear space, ensuring that its pressure remains at near normal environmental air pressure. The secondary function of the Eustachian tube is to drain any accumulated secretions, infection, or debris from the middle ear space.