This is the line that does the heavy lifting for from-sip-external
'_X.' => 1. NoOp(Received incoming SIP connection from unknown peer to ${EXTEN}) [extensions.conf:186]
See the _X., that will ONLY match a numeric sequence (or a string starting with digit, not sure). This was changed in version 17, because on 16, the line is:
'_.' => 1. NoOp(Received incoming SIP connection from unknown peer to ${EXTEN}) [extensions.conf:186]
This is a 17 bug, we’ve seen another case of this where _. was changed to _X. in 17. Open a ticket on github.