Is any of this based on current releases of FreePBX? The T38 options are configurable in the Asterisk SIP Settings.
So advising people that you added this in the Chan_SIP settings in the Extra Settings fields is wrong as there is already a spot in the GUI for these settings.
Also keep in mind that any version below FreePBX 13 is dead and EOL’d. No one is advised to stay on them and they should upgrade. At this point everyone should be upgrading to v14, it’s been out for 22 months now and by the end of the summer (I’m guessing) v13 will start down the path of being EOL’d.
Finally, Chan_SIP is not long in the tooth. It is set to be start being no-loaded (compiled but not activated) in upcoming new versions of Asterisk and will be removed completely at some point. So you should be learning how to do all this T38 stuff on Chan_PJSIP as that is the currently developed and supported SIP driver for Asterisk. Hopefully it doesn’t take you years to figure out now.