asterisk users - Dec 2009 - CDR is "NO ANSWER" when it should be "ANSWERED"

Hi,

I'm having trouble with dialing out on analog lines. Asterisk can't seem
to detect "answers".

I have two zap groups. 

Group 1 is connected to an external analog PSTN provider. This group seems to
work fine, especially with "answeronpolarityswitch".

Group 2 is a group of "GSM gateways", ie. devices that host SIM cards
so that you can dial from any PBX extension to another phone via a GSM SIM card
(supposedly, calls to cellular phones are cheaper this way). These devices act
just like any other analog line.

Every time I dial out through these analog GSM gateways the call gets through
just fine. However, if the call lasts more than the timeout set on the Dial()
application then it gets cut off. Also, if the call terminates, the CDR always
records "NO ANSWER" even if it actually has been answered. During the
call (answered and active), asterisk -rx "show channels" shows that
the calls made out through group 2 are in "Ring state".

Any ideas on how I can detect "answers" on analog GSM gateways?

example CDR:

"","7070","600000000","outbound","""TEST""
<7070>","IAX2/mgmbox-1278","Zap/6-1","Dial","ZAP/g2/600000000|300|TWf","2009-12-29
08:30:00",,"2009-12-29 08:30:10",10,0,"NO
ANSWER","DOCUMENTATION","1262075400.294",""

zapata.conf:

[trunkgroups]

[channels]
language=es
context=from-pstn
switchtype=national
signalling=fxs_ks
rxwink=300
usecallerid=no
cidsignalling=dtmf
hidecallerid=no
callwaiting=yes
usecallingpres=yes
callwaitingcallerid=yes
threewaycalling=yes
transfer=yes
canpark=yes
cancallforward=yes
callreturn=yes
echocancel=yes
echocancelwhenbridged=no
rxgain=6.0
txgain=2.0
group=0
immediate=no
answeronpolarityswitch=yes
hanguponpolarityswitch=yes
faxdetect=both

channel => 1-2

group=1
channel => 3-4

group=2
channel => 5-9

group=3
channel => 10-12
answeronpolarityswitch=no
hanguponpolarityswitch=no

Thanks,

Vieri