1. A method of switching an Instant Messaging (IM)-based voice call to a cellular-based voice call comprising the steps of: receiving in a service gateway a voice call destined for a telephone number, creating a first call record associated with the service gateway identifying the voice call, establishing over an IM voice network an IM-based voice call with an IM phone agent, including in the first call record first information about the IM-based voice call, connecting the voice call to the IM-based voice call, sending at least a portion of the first call record to a switchover agent, creating a second call record associated with the telephone number including the at least a portion of the first call record and second information about the IM-based voice call, establishing a cellular-based voice call and including information about the cellular-based voice call in the first call record, associating the cellular-based voice call with the IM-based voice call, sending a signal to a cellular phone agent indicating that the cellular-based voice call is for switch over purpose, and establishing the cellular-based voice call with the service gateway.