In order to change the SIP port for chan_pjsip from the default port 5060
to a custom value first go to Settings => Asterisk SIP Settings
Then go to the SIP settings [chan_pjsip] tab:
Now scroll down to the bottom of the page and look for
Change it to the desired port, e.g. 15060
click Submit on the bottom right
After that, don’t forget to click Apply Config on the top right
You also need to Restart Asterisk after the change.