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 Eustachian tube is used to equalize air pressure between the middle ear and the surrounding area, but it is also used to drain mucus from the middle ear. Keeping the pressure in a person's head equalized helps with balance and preventing dizziness.