Call transfer not working

Hello, everyone

I’m having problems with making call transfer work:

A calls B
A puts B on hold and calls C
B picks up the phone and all three can speak
PROBLEM: when B hangs up, A and C loose communication

I’m quite new to asterisk and even not sure if this scenario is possible (B hangs up, A and C keep talking). If it is a possible scenario, please could anyone direct me to a manual or give a hint where to start looking.

Thanks in advance