First, I should mention, in case you aren’t aware… You would only be able to do this for blind (unattended) transfers, because attended transfers are initiated by a regular call to the destination, and then the transfer happens later. So there’s no way to know when they’re initiating the call whether they’re calling to talk to the person, or to transfer a call.
Anyway, changing the ring tone is done by setting the Alert-Info SIP header in the dialplan. However, the value to place in the header depends on your phones. It should be specified in the administration or provisioning documentation for your phone model. You might have to set up the phones to allow it.
As far as the FreePBX setup for it, you would set the Alert-Info in two places, [from-internal-custom] (for transfers) and [from-internal-noxfer-custom] (override for non-transfers) in /etc/asterisk/extensions_custom.conf (Note: entirely untested code… may not work at all):
; Ring for transferred calls
[from-internal-custom]
exten => _X.,1,SIPAddHeader(Alert-Info: <transfer-ring-value>)
same => n,Goto(from-internal-additional,${EXTEN},1)
; Ring for non-transferred calls
[from-internal-noxfer-custom]
exten => _X.,1,SIPAddHeader(Alert-Info: <non-transfer-ring-value>)
same => n,Goto(from-internal-noxfer-additional,${EXTEN},1)
The one thing I’m unsure of is whether those Goto lines are going to work. According to the responses here, you would have to use a different extension. I’m not sure I believe that, but as I said, I haven’t tested this, so I’m still only guessing. Hopefully someone who knows for sure will chime in! Or you can try it and let us know. 