Hello, feixue.
To prevent unauthorized call forwarding, do as follows:
The call rights of enterprise users can be limited. If an enterprise user does not have the toll call rights, calls cannot be forwarded to non-enterprise users. Configure the call rights based on user requirements. For details about the configuration, see Allocating Numbers to SIP Users and Configuring the 32-Level Right Call Restriction under in the U1900 Product Documentation.
Properly configure the call forwarding services according to specific user requirements. Configure the call forwarding service based on user requirements. For details about the configuration, see Call Forwarding under in the U1900 Product Documentation.
Let me know if my solution works out for you. Thank you!