MetroPCS officially launched its prepaid service in 15 new cities today, expanding its footprint to cover 45 markets. But behind the scenes the carrier is also getting close to another milestone. It’s almost at the point where it can stop selling CDMA phones entirely and focus entirely on becoming a GSM carrier.

According to MetroPCS COO Tom Keys, 98 percent of all its dealers and stores are now exclusively selling GSM/HSPA and LTE devices that work on the network of Metro’s new parent company T-Mobile US(s tmus). The remaining 2 percent are in locations where T-Mobile’s network doesn’t completely overlap with Metro’s old CDMA footprint, Keys said. But once T-Mobile brings networks online in those areas, Metro will become entirely a GSM shop.

MetroPCS will still have to support existing customers with CDMA phones – T-Mobile won’t sunset the network completely until 2015 – but it’s already well on…