I am assuming that the box from Colt is an SBC (Session Border Controller) used only for VoIP. If it is also your gateway to the internet (for non-VoIP traffic), please provide details.
The usual setup is to have a second NIC on the PBX, cabled directly to the Colt SBC, set up with a static IP address given by Customer PBX IP. If you don’t have a second NIC but do have a managed or smart switch, you could set up Customer PBX IP on a VLAN and configure the switch to send that VLAN untagged to the Colt box.
If the COLT SBC Signalling Point is not on the same subnet as Customer PBX IP, please explain. If RTP Media IP is on a different subnet, you need to set a static route in the PBX (at the operating system level, not in Asterisk or FreePBX) to that subnet.
In Asterisk SIP Settings, set Local Networks to include the Customer PBX IP subnet and the RTP Media IP subnet, in addition to what’s there now.
I recommend a pjsip trunk with
SIP Server: (SBC Signalling Point)
SIP Server Port: 5060
To get started, set Outbound CallerID for the trunk to your main number (starting with +39) and set CID Options to Force Trunk CID. Set up a catch-all route to an extension for testing.
After you get it working, you can send different caller IDs and route different DDIs as desired.
If you have trouble, at the Asterisk command prompt, type
pjsip set logger on
make (or receive) a failing test call, paste the relevant section of the Asterisk log at pastebin.freepbx.org and post the link here.