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}
exten => 10,n,noop,_foo is: ${_FOO}
exten => 10,n,noop,__foo is: ${__FOO}