Methods and apparatus to measure market share for voice over Internet protocol (VoIP) carriers is disclosed. An example method includes querying a plurality of VoIP carrier servers to determine the VoIP carrier server that owns the telephone subscriber number and, in response to the querying, receiving a plurality of messages operable to determine whether the telephone subscriber number is found within any one of the plurality of VoIP carrier servers. The example method also includes, when the received plurality of messages is at least one of inconclusive or when the telephone subscriber number is not found within any one of the plurality of VoIP carrier servers, placing a first partial call to the telephone subscriber number from a first VoIP number within a first VoIP carrier network. Further, the example method includes, in response to placing the first partial call, receiving a first signal from the first VoIP carrier network, and based on the first received signal, determining whether the telephone subscriber number belongs to the first VoIP carrier network.