Cisco CP-8851 SIP drop connection when transfered

[Arogant]

Hello. This is my first time configuring a Cisco phone, so I kindly ask for your understanding. After many attempts, I managed to configure the phone to connect to the SIP provider (zadarma.com). Let's say everything works fine except for one thing.

I'm calling from phone A to phone B. Phone B forwards the call to phone C, and phone A gets disconnected. If I perform forwarding using #PhoneNumber#, then the forwarding works normally, and phone A is connected to phone C.

Is there a way to fix this? There are too many people in the company to forward calls using #number#. I'm not sure how to debug what's happening on the phone, so if needed, please assist me with that as well.

My SEP##########.cnf.xml file: https://pastebin.com/cF35dMtQ 

Best regards, Patryk

[Nithin Eluvathingal]

You need to check with your SIP provider.

[Arogant]

My SIP provider informed me that to forward a call, I need to press *NUMBER#. Unfortunately, I need to do this in a more automated way by clicking the "Transfer" button and selecting the number. This is because there might be a lot of call forwarding needed, and the person at the "front desk" won't be able to enter the forwarding combination manually each time.