Verifying an association between a phone number provisioned by a voice network and a phone device is useful on a synthetic communication network that reuses the phone number to transmit text and other media over a data network between devices registered on the synthesized network. Upon receiving a request to add a phone device registered with a voice network to the synthesized network, the phone number is presumed to be unconfirmed on the synthesized network. A verification message addressed to the phone device is transmitted through a data channel within the voice network. If the phone number is associated with the device, the message returns to the phone device over the data channel. If the message is received by the phone device, the phone number is confirmed as associated with the phone device, and the synthetic network authorizes communication between the device and other devices registered on the synthetic network.