Only if you are signing up for a service which your existing modem is not compatible with.
What does that mean?
For example, if you have an ADSL modem and sign up for one of our ADSL services then you shouldn't need a new modem.
If on the other hand you have an ADSL modem and sign up for one of our VDSL service, then you will need a new modem as ADSL modems are not compatible with VDSL services.
Another example would be if you are switching from a Cable based internet service to one of our DSL based internet services. Your existing cable modem would not be compatible with our DSL service.