asterisk users - Jan 2006 - cisco as5400, sip, asterisk. cisco won't detect that the call is answered

We've got this configuration :

Cisco as5400 --- asterisk main server ---- asterisk for cells ---- gsm 
gateway
cisco and the gsm gateway are connected to asterisk via sip, the two 
asterisk servers are connected via iax.
On a succesful call the cisco (not always, 60% of the times) will keep 
sending a ringtone to the connected phone, even if the call is answered, 
actually if the user behind the cisco talks the one after the gsm 
gateway will hear him, but not the contrary.
(like when you have a problem with  nat, plus the "I'm still hearing
the
ringingtone" problem)
((no, cisco is on a public IP, also the two asterisk servers, and all 
sip is canreinvite=no)

the dial chain is something like :

asterisk main server:
[cisco context]
X.,1,Dial(iax/cisco@asterisk for cells)

asterisk for cells:
[cisco context]
X.,1,Dial(sip/number@gsm gateway)


If the main server dialplan becomes like :

[cisco context]
X.,1,Answer
X.,n,Dial(iax/cisco@asterisk for cells)

the problem is solved, but all the calls are seen as answered by the 
cisco (well, they are) and this is not good for billing purposes.
(the 'asterisk for cell' server writes the cdr duration / billsec 
correctly, but trust is not of the business world ....)
(there are some lynksis paps connected to the asterisk main server, and 
they work perfectly)