The relevant paragraph of the Customer Agreement For Residential Services (which is basically your core contract with Comcast) which is 15(c) states:
c. Additional Representations and Warranties. . In addition to
representations and warranties you make elsewhere in this Agreement,
you also represent and warrant that:
i. Age: You are at least 18 years of age.
ii. Customer Information: You represent and warrant that you have
provided us with information that is accurate, complete and current,
including without limitation your legal name, address, telephone
number(s), the number of devices on which or through the Service(s) is
being used, and payment data (including without limitation information
provided when authorizing recurring payments). YOU AGREE TO NOTIFY US
IMMEDIATELY IF THERE IS ANY CHANGE IN THE INFORMATION THAT YOU HAVE
PROVIDED TO US, INCLUDING WITHOUT LIMITATION ANY CHANGE IN YOUR
TELEPHONE NUMBER OR MOBILE TELEPHONE NUMBER. FAILURE TO DO SO IS A
BREACH OF THIS AGREEMENT. IF YOU OWE ANY OUTSTANDING AMOUNTS FOR THE
SERVICE(S) OR HAVE ANY UNRETURNED EQUIPMENT, THIS OBLIGATION SHALL
SURVIVE THE TERMINATION OF THIS AGREEMENT AND SHALL CONTINUE UNTIL YOU
PAY ALL OUTSTANDING AMOUNTS IN FULL AND RETURN ALL EQUIPMENT. YOU
AGREE THAT YOU SHALL INDEMNIFY, DEFEND AND HOLD US HARMLESS FROM ANY
CLAIM OR LIABILITY RESULTING FROM YOUR FAILURE TO NOTIFY US OF A
CHANGE IN THE INFORMATION YOU HAVE PROVIDED, INCLUDING ANY CLAIM OR
LIABILITY UNDER THE TELEPHONE CONSUMER PROTECTION ACT (47 U.S.C. SEC.
227), AND ANY REGULATIONS PROMULGATED THEREUNDER RESULTING FROM US
ATTEMPTING TO CONTACT YOU AT THE MOBILE TELEPHONE NUMBER YOU PROVIDED.
The referenced statute prohibits robo-calls and junk faxes directed to someone with whom you do not have a pre-existing business relationship.
So, basically, Comcast is worried that if you don't keep your phone number updated and someone who is not a Comcast customer gets your phone number that they will violate the anti-robo call laws and be fined.
If that happens, you must pay all fines, damage awards and legal fees that Comcast incurs as a result of not keeping your contact information updated, resulting in punishments under this law or any other trouble Comcast gets in because your contact information is not updated.
Since not keeping your information up to date is also a "breach of this agreement" they can also terminate your Comcast services (paragraph 9) for failing to keep your contact information up to date (which can be enforced in a binding arbitration forum (paragraph 13) rather than a real court, even though those dispute resolution forums are notoriously unfair).
Of course, you are right to be skeptical. While errant robocalls may be the official justification for enforcing the policy strictly (and quite likely may be a term of a class action robocall lawsuit settlement of some kind), another primary reason that Comcast wants you to have up to date contact information is to facilitate debt collection if you don't pay your bill on time. This is supported by the following language in paragraph 15(c)(ii):
IF YOU OWE ANY OUTSTANDING AMOUNTS FOR THE SERVICE(S) OR HAVE ANY
UNRETURNED EQUIPMENT, THIS OBLIGATION SHALL SURVIVE THE TERMINATION OF
THIS AGREEMENT AND SHALL CONTINUE UNTIL YOU PAY ALL OUTSTANDING
AMOUNTS IN FULL AND RETURN ALL EQUIPMENT.
In practice, however, if Comcast doesn't get hit with fines for robocalling your old number, and is just requiring you to update information so that its debt collectors can pursue you more easily, your damages would be at most the cost that Comcast incurs for a skip-trace service to locate your new number so that it can harass you again until you pay your bill.