Blocking this merger is bad.
I'm a former T-Mobile UK employee, and a current Telefonica Shareholder and Employee.... I have no say in either corporation at this sort of level where deals exist.
Blocking this merger is a mistake.
T-Mobile US is a ticking timebomb. DT have been trying to get rid of it for ages, its weird 3G frequency allocation limits its value. Its lack of spectrum for some sort of service reasonable people would call 4G (HSPA is not 4G) limits its life expectancy.
Plainly put, T-MUS is going to go away; either DT are going to see more value in a shutdown and fire sale, going to see it die as it fails to compete against LTE networks, or it merges with a company that can get some value out of it.
Only one company reasonably can get value out of it - AT&T - the GSM space is value to AT&T today, the GSM handsets and SIMs may limit the work thats required tomorrow to migrate those customers onto the regular AT&T network (which the other major US CellCos can't boast).
This merger is the best thing that can happen to T-MUS for both DT and its users.