Outgoing calls on FreePBX, Troubleshooting

i’m a FreePBX newbie.

It’s a pleasure and a great opportunity for me to read many interested posts in this Open Source community.

A customer asked me to manage his FreePBX (it’s a FreePBX that recently reveal some troubles (I state that the FreePBX was configured by another technician that changed his job)

The FreePBX of my customer manage six Grandstream IP Phones : a few weeks ago half of these (three IP phones) have problems in outgoing calls (the remaining three IP phones work perfectly)

Activating the Asterisk log, I read many of the following messages :
[Mar 28 18:56:58] NOTICE[2966] chan_sip.c: Call from ‘205’ ( to extension ‘201’ rejected because extension not found in context ‘from-morelli’.

How can I start the debug to understand which FreePBX configuration file is misconfigured ? I have full access both of the SSH and WEB admin service.

Many thanks,