Comcast is tiptoeing into the wireless business to defend its broadband franchise. It may ultimately decide to jump in with both feet, with big implications for T-Mobile US.

The cable giant plans to launch a wireless service by mid-2017, using a combination of Wi-Fi and airwaves leased from Verizon Communications, Chief Executive Brian Roberts said at an investor conference Tuesday. The service, which will be marketed to existing customers as part of a bundle of services, could help Comcast retain customers. It could also...