Outgoing calls on FreePBX 2.9.0.12, Troubleshooting

Hi,
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 2.9.0.12) 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’ (192.168.1.86:5060) 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,
Marco