Microsoft store live chat said they pulled the Lumia 640 from their store website because they are not sure T-Mobile is going to carry the Lumia 640. What happened to Coming Soon?
From what I've read on a few web sites this week, June 17th you can order from T-Mobile for delivery and it will be available in select T-Mobile stores. Also expected in retail stores like Best Buy, Walmart, etc.on June 17th.
Also, T-mobile still has the phone listed on their website. Can't link but I have the page bookmarked and when you go to Google and type "lumia 640" the page for it comes up.
As brought up above, it's launching on T-Mobile online/telesales June 17th and to select other retailers. Which likely means Walmart as well since many have had tags for it in their stores.