A method of routing traffic through a mobile network, the method comprising: receiving (402, 502, 602), by a Session Border Controller, SBC,, a Session Initiation Protocol, SIP, invite message from a Mobile Virtual Network Operator, MVNO; identifying, by the SBC, the MVNO from the SIP invite message; determining (504, 604), by the SBC, a MVNO identifier, ID, for the MVNO; inserting, by the SBC, the MVNO ID in a private header of the SIP invite message; sending, by the SBC, the SIP invite message to a transit gatekeeper; receiving, by the transit gatekeeper, the SIP invite message from the SBC; extracting, by the transit gatekeeper, the MVNO ID from the SIP invite message; determining (506, 606), by the transit gatekeeper, whether the MVNO has permission to use the mobile network as a transit network based on the MVNO ID extracted from the SIP invite message, wherein the determining comprises accessing a Home Location Register, HLR, comprised in the mobile network, and using a lookup table stored in the HLR, the lookup table associating the MVNO ID with a plurality of conditions associated with use of the mobile network by the MVNO; in response to determining that the MVNO has permission to use the mobile network as the transit network, enforcing, by the transit gatekeeper, at least one condition of the plurality of conditions, the at least one condition being associated with attributes of an originating party or a destination party in a communication serviced by the MVNO; and allowing, by the transit gatekeeper, use of the mobile network in response to the determining that the at least one condition is satisfied, or denying, by the transit gatekeeper, use of the mobile network in response to the determining that the at least one condition is not satisfied. receiving (402, 502, 602), by a Session Border Controller, SBC,, a Session Initiation Protocol, SIP, invite message from a Mobile Virtual Network Operator, MVNO; identifying, by the SBC, the MVNO from the SIP invite message; determining (504, 604), by the SBC, a MVNO identifier, ID, for the MVNO; inserting, by the SBC, the MVNO ID in a private header of the SIP invite message; sending, by the SBC, the SIP invite message to a transit gatekeeper; receiving, by the transit gatekeeper, the SIP invite message from the SBC; extracting, by the transit gatekeeper, the MVNO ID from the SIP invite message; determining (506, 606), by the transit gatekeeper, whether the MVNO has permission to use the mobile network as a transit network based on the MVNO ID extracted from the SIP invite message, wherein the determining comprises accessing a Home Location Register, HLR, comprised in the mobile network, and using a lookup table stored in the HLR, the lookup table associating the MVNO ID with a plurality of conditions associated with use of the mobile network by the MVNO; in response to determining that the MVNO has permission to use the mobile network as the transit network, enforcing, by the transit gatekeeper, at least one condition of the plurality of conditions, the at least one condition being associated with attributes of an originating party or a destination party in a communication serviced by the MVNO; and allowing, by the transit gatekeeper, use of the mobile network in response to the determining that the at least one condition is satisfied, or denying, by the transit gatekeeper, use of the mobile network in response to the determining that the at least one condition is not satisfied.