A mobile device can send a notification to place a call using a PBX (e.g., using SIP signaling) or receive a notification of call availability for the mobile device at the PBX. An authentication token is provided over a data channel for use in authenticating the mobile device when calling the PBX on a voice channel. However, when the mobile device calls the PBX (e.g., on a DNIS number, where ANI information is made available to an application server), if ANI information is found to correspond to the mobile device, the PBX can send tone(s) indicating that the mobile device can respond with an abbreviated validation tone, rather than the entirety of the authentication token. If the ANI information is not obtained or not matched, then a different tone(s) is sent, indicating to provide the authentication token. If the authentication token provided does not match or if the mobile device does not respond, the voice channel is not set up.