How does this type of acquisition happen? How does the division of Microsoft that wanted this business determine that instead of trying to build it themselves,

How does this type of acquisition happen? How does the division of Microsoft that wanted this business determine that instead of trying to build it themselves, they should approach their potential competitor and ask to purchase it from them? I can understand most M&A, because at least one of the companies involved will be a whole organisation, but in this case a piece of Microsoft must have identified that they wanted to purchase a piece of AT&T. That seems really interesting.