T-Mobile has seen better days. First, it looks as though T-Mobile will be acquired soon by AT&T and to pour salt in their wounds apparently the subscriber results for Q1 2011 have been atrocious. However, T-Mobile is still doing all it can to win back its customers by revamping its prepaid plans to allow for unlimited data, launching cutting edge handsets, and now allowing unlimited calling over wireless networks.
TmoNews has gotten hold of a screenshot that shows that calls over wireless networks will be completely free from tomorrow (May 11th).
Right now you can make calls over a wireless network, which is useful where your data coverage is poor, but on the flipside, it cuts into your in-plan minutes.