Methods and apparatus to respond to monitor subscriber activity are disclosed. An example method includes receiving a first set of client telephone number (TN) data associated with a client and a current time period, and receiving a second set of client TN data associated with a prior time period. The example method also includes receiving a first set of subscriber activity data associated with the current time period, identifying a TN that is in the first set of subscriber activity data and is absent from at least one of the first or second sets of client TN data, and identifying the TN with at least one of a new inroad for a competitor or a new inroad for the client based on at least one activity code from the first set of subscriber activity data.