T-Mobile to Sunset MetroPCS's CDMA Network by 2015

It doesn't say anywhere that they are giving up on VoLTE. Long term that is the logical path, and is completely unrelated to maintaining or abandoning the CDMA network.

The CDMA network is only relevant in the decision of if they will be converting the GSM over to CDMA in the short run, convert the CDMA to GSM, or run parallel incompatible networks. Clearly the worst option is to run two networks instead of one.

In picking between CDMA and GSM the choice of GSM is an easy one. When you look at the combined customer base of T-Mobile and MetroPCS customers the customers currently on GSM will outnumber those on CDMA by more than 3 to 1 due to T-Mobile's much larger customer base. Also weighing in GSM's favor over CDMA is the favorable roaming agreement T-Mobile picked up from AT&T when their merger fell apart.

Since it would make no sense to change 3 1/2 times as many phones out and give up the AT&T roaming agreement, the combined company is migrating the CDMA customers to GSM and closing down the CDMA network while the LTE build out continues.