I can make and receive calls now. My SIP card was missing some config:
customs: []
And I added in pjsip.conf the follow lines:
local_net=192.168.31.0/24
local_net=127.0.0.1/32
external_media_address=mydomain.duckdns.org
external_signaling_address=mydomain.duckdns.org
Here’s my thanks to Jordy, @david551, and all the people at Discord (HA-SIP) and this forum. You guys are amazing.