Verizon Wireless' network.
In July, Comcast executive Greg Butz was promoted to spearhead the newly created Comcast Mobile division. It was also around this time when the company showed interest in the 600MHz auction held by the FCC. Last October, however, Comcast already alerted Verizon they were looking at selling mobile service under Verizon's networks. Both companies first entered into a partnership back in 2012.
In preparation with its MVNO service, Comcast will be relying on over 14 million Xfinity Wi-Fi hotspots throughout the country. These establishments include public hotspots in coffee shops, parks, restaurants, etc.; as well as routers in the homes of customers and an optional "guest mode" for sharing internet connections with those nearby.
As for information on pricing or detailed rollout plans, the Comcast CEO wasn't able to share more information. The only other info they divulged was that they hope to target their mobile service to its current cable customers. This is something Comcast hopes would enable them to address what AT&T is offering with its DirecTV satellite service and unlimited AT&T mobile data plan exclusively available for DirecTV subscribers.
Source: The Verge