Best way for Troubleshooting / debugging network problem?

Hi all,

Anyone here who might have some tips on what the best way is for troubleshooting?

Problem is that an extension outside my lan does sometimes work, and sometims not (not work as in “the extension is called, but not ringing”, or “the extension is picked up but no audio”)

The extension is in a hotel room, and always showing registered on the phone and on the asterisk sever.
CLI is showing the correct data. The only thing missing (sometimes) is the “-- SIP/6666-00000141 is ringing” (6666 is the extension nr.)

The strange thing is that sometimes it works in one go (calling to the extension) , or sometimes I need to keep trying!
Making calls from the extension in the hotel does not have any problems.

Not sure if the hotel is using a firewall, but if so could this be the problem?