IAX trunk and multiplex calls

Hello,
I have two sites with one server on each site. I have an IAX trunk between the two servers, this trunk use GSM codec and works. The problem is that each channel uses 50.0Kb (with GSM codec !!!) and there is no multiplex calls into the trunk. Can you help me with the configuration of this trunk?
Thanks.

here is the log when I call the second site,

Verbosity is at least 3
– Accepting UNAUTHENTICATED call from xxx.xxx.xxx.xxx:
> requested format = gsm,
> requested prefs = (gsm),
> actual format = gsm,
> host prefs = (gsm),
> priority = mine
– Executing [799@from-site1:1] Answer(“IAX2/site1-1”, “”) in new stack
– Executing [799@from-site1:2] Echo(“IAX2/site1-1”, “”) in new stack

here is the result of the ‘iax2 show peers’ command

server1*CLI> iax2 show peers
Name/Username Host Mask Port Status
site2 xxx.xxx.xxx.xxx (S) 255.255.255.255 4569 (T) (E) OK (303 ms)
1 iax2 peers [1 online, 0 offline, 0 unmonitored]

server2*CLI> iax2 show peers
Name/Username Host Mask Port Status
site1 xxx.xxx.xxx.xxx (S) 255.255.255.255 4569 (T) (E) OK (305 ms)
1 iax2 peers [1 online, 0 offline, 0 unmonitored]

and here is the configuration files iax.conf and extensions.conf which I use.

server1 :

iax.conf :

[site2]
type=friend
host=xxx.xxx.xxx.xxx
port=4569
auth=rsa
inkey=key_server1
outkey=key_server2
context=from-site2
qualify=yes
trunk=yes
trunkfreq=20
trunktimestamps=yes
nat=yes
encryption=yes
disallow=all
allow=gsm
jitterbuffer=yes

extensions.conf :

[from-site2]
exten => _0.,1,Dial(IAX2/site2/${EXTEN})
exten => _0.,2,Congestion

server2 :

iax.conf :

[site1]
type=friend
host=xxx.xxx.xxx.xxx
port=4569
auth=rsa
inkey=key_server2
outkey=key_server1
context=from-site1
qualify=yes
trunk=yes
trunkfreq=20
trunktimestamps=yes
nat=yes
encryption=yes
disallow=all
allow=gsm
jitterbuffer=yes

extensions.conf :

[from-site1]
exten => _0.,1,Dial(IAX2/site1/${EXTEN})
exten => _0.,2,Congestion