PJSIP supports a non-standard external port, but the FreePBX UI doesn't

Many people want to use non-standard ports externally, and have a NAT translate to the standard internally. CHAN_SIP allows this on the externhost parameter, where you can put the external address and port together. PJSIP has external_signaling_port available in the configuration files, but not in the FreePBX UI. 

Adding support for this parameter would enable users to easily add another layer of security to their installs.

There is a work around: use CHAN_SIP instead of PJSIP for external extensions. Or figure out which configuration file to add the parameter to, and edit it (this is sub-optimal because then your configuration is split between the UI and a custom configuration file)