In another sign competition between cable and phone companies may not be as robust as they would have you believe, Comcast last week announced a deal to sell local advertising on behalf of AT&T U-verse.

Under the agreement, Comcast’s Spotlight ad sales division will now sell local advertising slots on behalf of AT&T’s U-verse in cities where both companies provide service.

Comcast says the agreement will cover 21 cities, including:

• Atlanta
• Champaign-Springfield-Decatur (Ill.)
• Chicago
• Detroit
• Flint-Saginaw-Bay City (Mich.)
• Fresno-Visalia (Calif.)
• Grand Rapids-Kalamazoo-Battle Creek (Mich.)
• Hartford-New Haven (Conn.)
• Houston
• Indianapolis
• Jackson, Miss.
• Jacksonville (Fla.)
• Lansing (Mich.)
• Little Rock-Pine Bluff (Ark.)
• Memphis
• Miami-Ft. Lauderdale
• Monterey-Salinas (Calif.)
• Nashville
• Sacramento-Stockton-Modesto (Calif.)
• San Francisco
• West Palm Beach-Ft. Pierce (Fla.)

This leaves Comcast with a lock on local ad sales for both its own cable and AT&T’s U-verse systems. Most major cable networks offer their affiliates opportunities to insert local commercials during certain advertising breaks. The ad insertions provide a lucrative revenue stream for pay television providers, which can target viewers of specific cable networks or run the same messages across hundreds of cable channels.

The deal will save AT&T from having to hire additional employees to handle local ad sales and will let advertisers cover one or both systems in a single ad buy.

But critics wonder if Comcast cooperating this closely with AT&T is good for competition.

The deal is not unprecedented, however. AT&T has also partnered with Charter Cable to cross market local ad sales in cities where U-verse and Charter compete head to head.