Not all CDMA phones will work in all Metro markets.
Some Metro markets use AWS frequencies, instead of "traditional" CDMA frequencies. Metro is in 14 markets and 5 or 6 use AWS -- like New York city. You cannot flash a Verizon/Sprint/Virgin phone to Metro and have it work in those markets since they don't have the necessary frequencies baked into the chipsets. Phones manufactured for MetroPCS have the frequencies.
So, even if you live in a good market, if you then travel to an AWS market your phone won't work.