If you're dealing with a traditional TDM setup, then you will have to delete channels of bandwidth to add channels of voice.

If you are dealing with a VoIP T1, then it could be a full T1 of Internet; when a phone call is made, the bandwidth is dynamically assigned to the phone call. The Internet would slow down some, and then pick back up once the phone call is complete.

Your service provider would have to tell you if it is a TDM or VoIP T1.