Chanspy SPYGROUP

Hi everyone,
i'm new to trixbox and i have a problem with a costumer who want to be able to spy 4 diferent extensions in my company.
Searching i realized that i have to set ${SPYGROUP} variable but i cant find how.

This is my extensions.conf

[from-pstn]
exten => 1461,1,Answer
exten => 1461,n,Wait(1)
exten => 1461,n,ChanSpy(SIP/1481)
exten => 1461,n,ChanSpy(SIP/1485)
exten => 1461,n,ChanSpy(SIP/1483)
exten => 1461,n,ChanSpy(SIP/1493)
exten => 1461,n,Hangup

It only working on the first extension and when i press * it come back to the first again.
My question is how to set de ${SPYGROUP} variable and how to define a group/content of a group.

Thanks in advance.