Which number from the ring group answered the call?

I’m dialing a group of numbers from the AMI using originate:
action: originate
channel: SIP/trunk/11111&SIP/trunk/22222&SIP/trunk/3333
context: ringgroup-answered
exten: s
priority: 1

I’m doing that so when one of the numbers answer the call it will stop ring the other extens,
all is working well, but when the call ends I don’t know which exten answered the call,
I except to get the variable on the context ringgroup-answered so I can update my cdr correctly,
I’m using dumpchan() like that:
exten => s,1,dumpchan()
but none of the variables describe who answered the call,
Any idea anyone?

I didn’t realise that originate accepted a full dial string!

However, the answer is that ${CHANNEL} contains this information.