Under the deal, Sprint will handle the connectivity between traditional phone providers and Comcast's IP-enabled Digital Voice service "in select markets." Comcast plans to launch VoIP in 20 markets by year-end.
The deal could end up becoming Sprint's largest with a cable operator. Before Comcast, its highest profile deal was with Time Warner Cable , which has tapped Sprint and MCI to deploy VoIP virtually across the board. Sprint also has VoIP relationships with Mediacom Communications , Charter Communications  and Cablevision Systems Corp. 
The formal announcement with Sprint gives even more visibility into Comcast's technology plans for VoIP. On Wednesday, the MSO announced separate VoIP managed backbone agreements with Level 3 Communications Inc.  and AT&T Corp. 
The agreement between Sprint and Comcast could also be seen as a step toward a wider partnership that involves mobile voice services - a coveted, but still missing, component of cable's service arsenal. Wireless voice service is not a component of the deal announced Thursday, however.
Sprint and Time Warner Cable have already disclosed a wireless trial in the Kansas City area. Kansas-based Sunflower Broadband  also resells Sprint mobile services.