It should happen automatically.  Is A behind NAT?  Sounds like a firewall/NAT may be dropping the bye.  Also, have you done a debug on FS to see if it actually sends a bye or not to A?

From: freeswitch-users-bounces@lists.freeswitch.org [mailto:freeswitch-users-bounces@lists.freeswitch.org] On Behalf Of Simon Leck
Sent: Tuesday, September 06, 2011 11:42 AM
To: 'FreeSWITCH Users Help'
Subject: Re: [Freeswitch-users] - freeswitch bye issue

Hi Everyone, kindly help if you can

At the moment I did encounter a issue, A leg user called a b leg user and when b leg user hanged up (Bye is not send by freeswitch to the A leg user)  the only way I could hanged up this call  to hanged up the A leg user manually? Would be great if someone could enlighten me on how this can be achieved?

Thanks in advanced to anyone for helping me out

Thanks again

Simon Leck