asterisk users - Mar 2006 - get no connection, very often, but not allways, why?

Hi,

I have an ISDN phone connected to a hfc-s card. I use it to phone via
an iax provider to foreign countries. Inside my country it works reliable,
but to other country it happens very often that the other side hears ringing
and before it can take the phone the line is dropped. What makes me
wonder is that I hear no ringing at all. With asterisk -vvvvc I get this:

Asterisk Ready.
*CLI>   == Primary D-Channel on span 1 up for TEI 64
    -- Accepting overlap voice call from '' to
'<unspecified>' on channel
0/2, span 1
    -- Starting simple switch on 'Zap/2-1'
    -- Executing Dial("Zap/2-1",
"IAX2/<user>:<password>@sip.coco-connect.de/XXXXXXXXXXX")
in new stack
    -- Called <user>:<password>@sip.coco-connect.de/XXXXXXXXXXX
    -- Call accepted by 62.180.50.221 (format g729)
    -- Format for call is g729
    -- Channel 0/2, span 1 got hangup
    -- Hungup 'IAX2/62.180.50.221:4569/1'
  == Spawn extension (extern, XXXXXXXXXXX, 1) exited non-zero on
'Zap/2-1'
    -- Hungup 'Zap/2-1'

Does the line "-- Channel 0/2, span 1 got hangup" mean that the
ISDN-Phone
drops the line first?
If yes, could it be, because the phone gets no ringtone a too long time?
I use no timeout for this channel.
Thanks for any help.

Regards
Gerald