Have you installed a Sametime SIP Edge Proxy in your DMZ.
The IBM Lotus SIP Edge Proxy is a SIP Application installed over a WAS server.It connects external clients to the Sametime SIP Proxy and Registrar.
http://www-10.lotus.com/ldd/stwiki.nsf/dx/Installing_a_Lotus_SIP_Edge_Proxy_Server_st852