Missing ${EXTEN} working with IAX members in Queues

Hi guys! I really think I need help.
This is my problem:
I have multiple iax trunks registered on an Asterisk 1.4.27
Multiple incoming calls from different PSTN lines (so different called numbers) have to fire a linear ringing strategy for that IAX trunks.
If the first trunk is unavailable or busy, the incoming call must be routed to the second trunk and so on.
I wrote a dialplan this way:

[incoming]
exten => _linenumber1,1,Dial(IAX/Trunk1/${EXTEN},5)
exten => _linenumber1,2,Dial(IAX/Trunk2/${EXTEN},5)
exten => _linenumber1,3,Dial(IAX/Trunk3/${EXTEN},5)
exten => _linenumber1,4,Hangup
exten => _linenumber2,1,Dial(IAX/Trunk1/${EXTEN},5)
exten => _linenumber2,2,Dial(IAX/Trunk2/${EXTEN},5)
exten => _linenumber2,3,Dial(IAX/Trunk3/${EXTEN},5)
exten => _linenumber2,4,Hangup
exten => _linenumber3,1,Dial(IAX/Trunk1/${EXTEN},5)
exten => _linenumber3,2,Dial(IAX/Trunk2/${EXTEN},5)
exten => _linenumber3,3,Dial(IAX/Trunk3/${EXTEN},5)
exten => _linenumber3,4,Hangup

I need to pass the ${EXTEN} because also the receiving trunks have to route calls on a DIALLED-NUMBER basis
Everything works fine, but now i need to manage many incoming line numbers and many IAX trunks, so I decided to use Asterisk Queue.

This is my setup for queues:

In queues.conf I put

[TrunkPool]
wrapuptime=10
timeout=10
strategy=rrmemory
member= IAX2/Trunk1/${EXTEN},1
member= IAX2/Trunk2/${EXTEN},1
member= IAX2/Trunk3/${EXTEN},1
maxlen=1
leavewhenempty=strict
joinempty=strict
ringinuse = no

In extensions.conf i put

[incoming]
exten => _linenumber1,1,Queue(TrunkPool)
exten => _linenumber1,2,Hangup
exten => _linenumber2,1,Queue(TrunkPool)
exten => _linenumber2,2,Hangup
exten => _linenumber3,1,Queue(TrunkPool)
exten => _linenumber3,2,Hangup

In this way remote trunks ring according to linear strategy, but they don’t receive the dialed number. Instead, them receive literally the string “${EXTEN}” and not the variable value.
If I omit the /${EXTEN} in the above member’s definition whitin queues.conf, the remote trunks receives “s” as extension value.
Of course I’m doing something wrong, but I’m going crazy and I really need help.
How can I pass to IAX trunks the dialled number of an incoming call if I work with queues?