following my own post...
It seems I do not understand correctly which 'context' to use.
Is this a correct way of setting context:
Peer side -> context=mainmenu
User side -> context=<cluster_name>
-- Accepting AUTHENTICATED call from 209.217.98.254:
> requested format = ulaw,
> requested prefs = (ulaw|gsm|adpcm),
> actual format = ulaw,
> host prefs = (ulaw),
> priority = mine
-- Executing [5149079086@internal:1] AGI("IAX2/5149079086-5", "selintra|OutCluster|5149079086") in new stack
-- Launched AGI Script /var/lib/asterisk/agi-bin/selintra
-- AGI Script selintra completed, returning 0
-- Sent into invalid extension '5149079086' in context 'default' on IAX2/5149079086-5
-- Executing [i@default:1] PlayTones("IAX2/5149079086-5", "congestion") in new stack
== Auto fallthrough, channel 'IAX2/5149079086-5' status is 'UNKNOWN'
-- Executing [h@default:1] Hangup("IAX2/5149079086-5", "") in new stack
== Spawn extension (default, h, 1) exited non-zero on 'IAX2/5149079086-5'
-- Hungup 'IAX2/5149079086-5'