[asterisk-users] Asterisk not replacing private FROM ip with public IP in INVITE

[TTT]

I've split this thread off from another (PJSIP authentication) because I think the root cause is something different.    I think the problem is the following FROM line in my SIP INVITE transaction:

From: "MYNAME" <sip:16667778888 at 172.31.253.4>;tag=773a3e6a-a677-4fb1-95fc-54b379b650a4

The IP address above is an internal/non-routable IP, so Twilio is rejecting it.  For some reason Asterisk is not replacing the private IP with my public IP address.  My pjsip.transport.conf contains a stanza for this transport with:

local_net=172.31.0.0/16

Is that all that's needed for Asterisk to replace the from IP with the external IP?  I'm not clear on why Asterisk is not substituting the private FROM ip with a public one...