There can be many problems why it doesn’t work. I will describe to you the most common ones I have encountered.
Just as the other person writes, it is necessary to set the correct caller ID. The pbx automatically changes the caller ID on an incoming call to the caller ID. The Yeastar S series pbx even have the option to set which identification you want to set when forwarding calls, whether Transferor or Transfree, but only for Attended transfer during an ongoing phone call.
The second problem is voice channels. For the needs of call forwarding, it is necessary to have at least two voice channels at the SIP Trunk provider.