It looks like the Lumia 640 may be coming to T-Mobile US a bit later than some had hoped. While the affordable mid-range phone is now available in the US through Cricket, it looks like the 640 will arrive on the "Un-carrier" on July 16.
This news comes from T-Mobile's own Twitter account, responding to a question about availability of the Lumia 640.
@WaverunningGeek Hey, good news! It looks like that device is going to released via all channels on 7/16! *PKB— T-Mobile USA (@TMobileHelp) May 15, 2015
It should be noted that T-Mobile and Microsoft have made no formal announcement about the Lumia 640's availability at the carrier, so this date may change at some point in the future.
A listing on the Microsoft Store late last month indicated that the T-Mobile version of the Lumia 640 would be arriving "soon," with a price listing of $89. It is currently available from Cricket Wireless starting at $129.99 without a contract.
Source: T-Mobile (Twitter)