Alphanumeric Paging Entry Protocol: http://www.l0pht.com/pub/blackcrwl/commcom/datacom/ixo.html, a brief document that outlines the IXO protocol.
IXO Mailing List FAQ: ftp://ftp.netcontech.com/pub/paging_info/ixo_faq.txt, a FAQ about an IXO mailing list and the IXO protocol, all wrapped into one.
Pager Hardware Reprogramming and Paging Protocols: http://199.201.145.2/~oblivion/blkcrwl/cell/pager/pager.html. For those who like to get into the guts of pagers, here's some more information on paging protocols, pager programming and pager hardware.