In the early 2000's I wrote a Visual Basic application that resulted in alphanumeric pages over the air. The software was in two parts. The "client" user interface was where one composed their message and chose the pager(s) recipients. A short text string was assembled containing the message, addressee common name, message priority and time stamp. That text was sent over a network and written to a common file residing on the "server". The client-server model was utilized to accommodate multiple remote client terminals. The server software prioritized unsent messages, located the recipient pager's address in a table and formatted and sent a command string to the paging hardware. It also logged the transactions. The paging hardware consisted of the Kantronics 9612 "Packet Communicator" connected to a Kenwood TK-780 transceiver. The Kantronics modem receives commands via a serial port and generates POCSAG tone data for transmission.
I think it would be relatively simple to parse an inbound email into the segments needed to format a page command for a hardware modem like the Kantronics 9612. Using a soundcard or other software modem is more of a challenge.