The proposed framework describes a service for users that gives them the ability to make gsm calls from their smartphones without revealing their identity. The principle to achieve that is simple: instead of using your cell phone to make a call, pick somebody else’s phone to do so. We proposed an infrastructure of smartphones, sip registrars and sip proxies to provide caller anonymity. We developed a testbed where a smartphone registers on a SIP registrar and can start GSM conversation through another smartphone acting as a GSM gateway, by using a SIP proxy. Empirical evaluation revealed no significant QoS degradation.