Snom phone drops forwarded calls

I'm posting the same questions to the 3CX forum. I'm not sure if the breaking issue is with the 3CX configuration, the Snom phone, or perhaps some firewall rule even.  Note - I'm new to VoIP in general. Please ELIF5 and let me know what other information I can gather for you.

We are having trouble with phone forwarding since the 3CX v20 upgrade. 

Situation and Use Case:
The front receptionist desk for an organization that we monitor has two seats/phones. Both phones are SnomD785-SIP running firmware version 10.1.175.16. They are both configured in 3CX to ring on calls from either of two "main" extensions (100, 101). I believe this is done via a ring group containing 100 & 101, but I'm not entirely sure about that. These are the outward facing lines for accepting external calls.

Before the upgrade, the receptionist would simply put the Snom on call forwarding to another extension (113) while away from their desk.  It's possible this was never officially supported by 3CX, but it worked before.

Problems:
-Since the V20 upgrade, when forwarding from the Snom, the calls will forward to 113 but drop immediately when answered. -A second issue is that the person answering 113 needs to know that the call is coming form the main lines. So the inbound caller id should be 100, 101 instead of the inbound caller's actual number. This apparently was the case before (not sure, I suspect maybe not). At any rate, that's what they want now. 

Desired end-state:
-The receptionist should be able to quickly put the main lines (100, 101) into a forwarding mode that will ring to 113. -This is preferably done by pressing a button on the Snom phone (e.g. Call Forward, or DnD), but the receptionist can also change status of the extensions from their mobile app.
-They should not have to log into the web console (e.g. to change ring group members).
-When a forwarded call rings to 113, it should display the 100/101 as the inbound caller ID (I'll be happy to just get forwarding working for a start).

Solutions attempted:
-We monitor this account remotely. Snom phone forwarding works in our office (same Snom model running v10.1.84.14), but without changing inbound caller ID to the extension being forwarded.
-We have tried several iterations of configurations involving DnD status on the phone and/or in 3CX, and/or ring groups.
    -e.g. DnD status set to forward to 113; DnD status set to call a ring group including 113; and several other combinations.
-The closest we got was getting internal calls to forward (without changing caller ID), but inbound external calls still did not forward. 
-Upgraded to latest firmware - no change. 

We would prefer to have the old setup where the receptionist simply pushes the call forward button on the Snom, but we'll take any ideas that are not complicated to toggle. Thanks in advance!