We have Asterisk 14.4.1 and 10 Grandstream GXP1628 phones with latest firmware (18.104.22.168), all phones are behind NAT and connect to server via public internet.
5 phones work without problems, they are connected via provider X.
Other phones are using other (different) providers, and they go into unreachable state about 20 times per day. It can last between 5 minutes to 5 hours before they become reachable again.
All phones have in their settings active NAT: Keep-alive and NAT send Options every 15 seconds. Asterisk also has qualify set to 15 seconds.
How can I debug the reason for going offline, and how can I solve the issue?