Transparently routing a telephone call between mobile and VOIP services

Systems and methods are provided for routing a telephone call intended for a communications device between a mobile network and a VOIP service, where the mobile network and VOIP service may be connected through the PSTN. The VOIP service may receive telephone calls and may direct the telephone calls to the communications device through the Internet when a stable Internet connection is present, and may route telephone calls to the mobile network through the PSTN otherwise. When a call is routed to the mobile network, the mobile network may make the call the communications device to establish a telephone connection through a cellular link. While a telephone call is in progress, the VOIP service and communications device may be configured to seamlessly switch the telephone call to a different service depending on the status of the communications device's Internet connection.