Dear Maintainer,

I have linphonec installed on my Ubuntu 14.04, and I have a SIP account
registered from my AVM FRITZ!Box 6360.

When I now try to perform a "Blind Transfer" using
"transfer $call-id $phonenumber", the transfer fails and I get the SIP error
code "429 Provide Referrer Identity" returned from the Fritzbox.

I suppose this is because of linphonec not sending the "Referred-By"-header.

Is it possible to fix this bug so FRITZ!Box users can use the Blind Transfer?