A method of learning the associations between predefined service provider network identifiers used to identify individual ones among a plurality of wireless devices in a Machine Type Communication, MTC, service provider network and corresponding external identifiers that are assigned to the wireless devices by an access network, for use by the MTC service provider network in identifying individual ones of the wireless devices to the access network, wherein the MTC service provider network is external to the access network and the wireless devices are configured to access the MTC service provider network via the access network, and wherein the method is carried out by a network node configured for operation in the MTC service provider network and comprises: receiving, from the access network, an external identifier that has been assigned by the access network to a particular one of the wireless devices among the plurality of wireless devices, wherein the network node does not know to which wireless device the access network has assigned the external identifier; storing data establishing a linking between the external identifier and a token, such that the external identifier is uniquely paired with the token; triggering the wireless device to establish a data session with the access network for use in initially registering for a service provided by the MTC service provider network, based on sending triggering signaling toward the access network that identifies the wireless device via the external identifier and includes the token for forwarding to the wireless device; receiving registration signaling sent from the wireless device via the access network, said registration signaling sent responsive to the triggering signaling sent from the MTC service provider network; and responsive to recognizing that the registration signaling includes the forwarded token, performing a binding operation comprising: obtaining the service provider network identifier of the wireless device from the registration signaling sent from the wireless device; accessing the stored data corresponding to the forwarded token, to determine the linked external identifier that was previously uniquely paired with the token; and binding the linked external identifier to the service provider network identifier of the wireless device, so that the service provider network node knows which external identifier to use for subsequently initiating communications with the wireless device via the access network. receiving, from the access network, an external identifier that has been assigned by the access network to a particular one of the wireless devices among the plurality of wireless devices, wherein the network node does not know to which wireless device the access network has assigned the external identifier; storing data establishing a linking between the external identifier and a token, such that the external identifier is uniquely paired with the token; triggering the wireless device to establish a data session with the access network for use in initially registering for a service provided by the MTC service provider network, based on sending triggering signaling toward the access network that identifies the wireless device via the external identifier and includes the token for forwarding to the wireless device; receiving registration signaling sent from the wireless device via the access network, said registration signaling sent responsive to the triggering signaling sent from the MTC service provider network; and responsive to recognizing that the registration signaling includes the forwarded token, performing a binding operation comprising: obtaining the service provider network identifier of the wireless device from the registration signaling sent from the wireless device; accessing the stored data corresponding to the forwarded token, to determine the linked external identifier that was previously uniquely paired with the token; and binding the linked external identifier to the service provider network identifier of the wireless device, so that the service provider network node knows which external identifier to use for subsequently initiating communications with the wireless device via the access network. obtaining the service provider network identifier of the wireless device from the registration signaling sent from the wireless device; accessing the stored data corresponding to the forwarded token, to determine the linked external identifier that was previously uniquely paired with the token; and binding the linked external identifier to the service provider network identifier of the wireless device, so that the service provider network node knows which external identifier to use for subsequently initiating communications with the wireless device via the access network.