Although T-Mo has recently switched over to IMS wifi-calling from Kineto, there seem to be issues with IMS as well. The biggest complaint on the Kineto system (used on the Defy and others) is simply poor quality. So, go into your router's setup menu and see if the default under wireless settings is "mixed mode". Mixed mode attempts to allow both G and N compatible devices to connect; but it's a compromise. I set my office router to N mode and my home router to G mode and now my wifi-calling is fantastic.