Can't transfer extension

I was testing the sip trunk function of FREESBC recently. The inbound and outbound functions of the direct line call are normal, but when I switch from the automatic switchboard to the extension, it will automatically hang up.
After capturing the packet, the analysis is that after the DTMB key dialing information is not forwarded'invite' to the operator, the call cannot be established normally, and the internal device actively hangs up.
I also tried to modify the settings in Call Transfer Options, but it still doesn't seem to improve the results.

Chinatel→freeSB（NAT）→IPPBX

Hi Cheng514,

For this kind of issue which may involve further investigation, I suggest you considering our ProSBC package.

I also see you had sent us email about this topic, I would reply the same to you.

Best regards,

William