How to fix UNREACHABLE issue for softphone connected to Asterisk?

Question

Asterisk

When you turn on the Zoiper softphone, the peer writes the status "OK" in the console. After a couple of minutes, the message "Peer '765' is now UNREACHABLE! Last qualify: 17" appears, and accordingly the calls do not go through. The softphone is connected via an external network. Here are the peer settings for it:
[765]
host=dynamic
type=friend
secret=*******
qualify=yes
nat=force_rport
port=****
directmedia=no
context=something-internal
disallow=all
allow=ulaw
allow=ulaw
username=765

Also the status sometimes changes back to "OK" and restarting the softphone helps.

Answer 1

Figured it out myself. I tried using softfon zoiper, and there it was necessary to enable "Keep alive time-out" in the advanced settings, and it all worked.