In a statement T-Mobile provided to the folks at Windows Central, “We work closely with phone manufacturers, including Microsoft, to continually adjust our device portfolio based on supply and customer demand. As a result of this, we no longer have inventory to support taking online orders for the Lumia 640 – and Microsoft will not be providing us with additional inventory.”
However the carrier also notes, “We do have limited quantities of the Lumia 640 still available in select T-Mobile stores – customers can call their local store to check availability.” This means that if you absolutely want the Lumia 640, there is a chance you could walk into a T-Mobile store and buy one there, but like the carrier states, you might want to call ahead first.
It is a pity that the phone is no longer offered by T-Mobile, but as Windows Central points out the handset is still available via alternate sources such as Walmart, AT&T, MetroPCS, and Cricket Wireless.