A database stores unique associations between account identifiers and mobile-network telephone numbers assigned by a mobile network to mobile communications devices. The account identifiers are also associated with voice-over-IP (VoIP) telephone numbers. Voice calls between the mobile communications devices are made as voice-over-IP (VoIP) calls using respective VoIP telephone numbers. Tracking of the VoIP calls is performed through the mobile network using respective mobile-network telephone numbers, so that a prepaid balance can be enforced.