In the relevant transport section in pjsip.conf (or for chan_sip, the general section in sip.conf). NB these files are managed by FreePBX and you should change them through the GUI, or, if using custom transports, for chan_pjsip, in the relevant custom .conf file.
Can it be configured in any way so that when the internet provider is changed, the ip updates automatically? or do I need to come here to change it manually?