A phone number only messaging experience is provided in an Inter-Carrier Messaging System service. Subscriber to subscriber digital messages containing user data are routed between carrier's networks using an Inter-Carrier messaging module with appropriate MIN/carrier/carrier routing look-up capabilities. Digital messages containing user data (e.g., short messages, email, digital messages in general, etc.) may be sent to a subscriber in another carrier's network addressed only with a phone number. If the recipient is outside of the sender's carrier network, the Inter-Carrier messaging module of the Inter-Carrier service provider (ICSP) determines the appropriate carrier for the recipient, appends the appropriate syntax to the digital message containing user data to allow internet protocol (IP) or other standardized communication techniques between message service centers of the two carriers, and routes the digital message containing user data to the destination carrier. The recipient in the other carrier's network may reply back to the ICSP's sender using only the subscriber's directory number.