(integer, 1 - 65535) The TCP port which the SMPP server listens on may be specified with either this option or the port channel keyword. This port number must be specified through either of these two mechanisms. If it is specified with both mechanisms, then the setting made with the SMPP_PORT option takes precedence. Note that there is no default value for this option.
For two-way SMS, make sure its the same port as the LISTEN_PORT for the SMPP relay.