A method for routing calls between a third party telecommunications device (“TD”) and a subscriber TD associated with a primary service and a second line service (“SLS”) involves associating the SLS number of the subscriber, the primary number of the subscriber and the primary number of a third party via a common relationship number. Calls directed from a third party to the SLS number of a subscriber are routed to an SLS platform and redirected to the subscriber TD. Calls directed from the subscriber TD to the third party use the relationship number to route the call to the SLS platform. The combination of the primary service number and the relationship number identifies the third party number for call completion. Calls can be directed to and from an SLS number of a subscriber TD using a combination of protocols such as ISUP, CAP and SIP.