dialplan:

machine 1:

exten => 2826781,1,set(__FOO=bar)
exten => 2826781,n,noop,foo is: ${FOO}
exten => 2826781,n,Dial(IAX2/pablo@pablo/10,60)

machine 2:
exten => 10,1,noop,foo is: ${FOO}

debug output:

machine 1:
    -- Executing Set("CAPI/contr1/2826781-96", "__FOO=bar") in new stack
    -- Executing NoOp("CAPI/contr1/2826781-96", "foo is: bar") in new stack
    -- Executing Dial("CAPI/contr1/2826781-96", "IAX2/pablo@pablo/10|60") in new stack

machine 2:
    -- Executing NoOp("IAX2/62.143.74.253:4569-1", "foo is: ") in new stack