Just installed a new cable modem. As required, I had to contact my ISP with the unit’s serial numbers in order for them to activate the modem. Why is this necessary?
To ID you. They can see the ID the unit sends out.
Perhaps you also have to login, and you might then use someone elses login. (eg if you were out of quota but they were not.) They may clamp down on sharing logins around like that if there was a problem …anyway they can physically ID you even if you use someone elses username/password.
To prevent theft of services. It is really easy to plug an extra cable modem into their network. Without a way of matching each modem to a particular paying customer, they’d end up with a lot of non-customers leeching off their service. Even paying customers might add a 2nd modem to get more bandwidth or to avoid identification when doing skeevy downloads.
Any cable modem whose ID doesn’t match what is in their database gets blocked.
Also keep in mind that there can be tiers of service. For a given monthly fee you get so much bandwidth and/or monthly gigabytes. They have to know who you are in order to set the modem to match the right plan.
(For us, an unauthorized modem only gets you to a login page. No need for a phone call. Once logged in, the new modem is authorized and the old one blocked.)
Charter started charging $5 a month for their cable modems. So I bought my own. The instructions said to contact the ISP and I called Charter. They said it wasn’t necessary for them. This was about 4 years ago. Since then Charter has started providing their cable modems for free again.