Doesn't send to voice mail when on call

[S-F]

I have configured 3CX to send calls to voice mail if the line is busy but callers get a busy signal. I am still experimenting with 3CX so this is the only account I have running on the PBX. Is this due to not having a spare SIP channel for the voice mail?

 

[S-F]

Is this issue not exclusive to the Skype trunk? Should this thread be moved to the general forum?
Has no one else had this issue?