[Asterisk-Users] Inheriting variables

[Asterisk]

I'm trying to set a channel variable and make it available to another 
channel:

I thought that if I SetVar(_SomeVariable=SomeValue) or 
SetVar(__SomeVariable=SomeValue) then SomeVariable would be available in 
the destination channel.

However __SomeVariable, _SomeVariable and SomeVariable are all blank.

The scenario:

Agents logon to the queue using callbacklogin. From what I can gather 
the following happens:

1) Incomming Call -> answered, plays welcome message added to Q
2) Local channel calls the agent, plays announcement
3) incomming call connected to agent

where would I be able to retreive the inherited variables ?