Patent ID: 8909224

Claim:
A method comprising: receiving, by a user device, an Internet Protocol address from a radio access network; establishing, by the user device, a first connection with a first Internet Protocol multimedia subsystem (IMS) core of a first carrier network of a first carrier using the radio access network and the received Internet Protocol address, wherein the first carrier issues a first phone number used by the user device; establishing, by the user device, a second connection with a second IMS core of a second carrier network of a second carrier using the radio access network, the received Internet Protocol address, and the first carrier network, wherein the second carrier issues a second phone number used by the user device; receiving, by the user device, notification about a first call from the first carrier network; determining, by the user device, whether, at a time when the notification is received, the user device is using the second carrier network for a second call; and transmitting, by the user device, a busy message to the first carrier network when, at the time when the notification is received, the user device is using the second carrier network for the second call.