T-Mobile CEO John Legere sent out a tweet earlier this morning which said that calls are now "operating normally nationwide." The executive did not mention what caused the problems for T-Mobile customers. Complaints started rolling in on Friday evening and after dissipating overnight, they picked up again heading into Saturday evening.
If for some reason you are still having issues making calls over the T-Mobile network, power cycling your device might be able to clear up the problem.
source: @JohnLegere, T-Mobile