Best way to get Comcast promotions?

I recently helped a friend, who is a long-time subscriber to Comcast basic cable, upgrade to digital cable and internet service. I was told by one CSR (after pulling up friend’s account) that I could get digital starter service for $29.95 and internet for $19.95 for 6 months. After I discussed this with my friend we called back and was told by another CSR they couldn’t offer that promo because we weren’t going up a tier in service, the account would be flagged, and after review we wouldn’t get the promo anyway. Can accounts get flagged if a CSR does a specific promo? How does Comcast customer service work, or more specifically, how do you get the best deals?