My asterisk will stop

Hi all,

If I make a call to a PSTN phone which is busy, I will get the following errors

– Called ISDN1/95145235
[Jun 11 18:59:52] WARNING[11554]: chan_capi_utils.c:305 capi_wait_conf: ISDN1#28: timed out waiting for CONNECT_CONF
== Spawn extension (sip-ser, 006595145235, 1) exited non-zero on ‘SIP/’
[Jun 11 19:00:10] WARNING[11489]: chan_sip.c:1900 retrans_pkt: Maximum retries exceeded on transmission 118155927711908.fifouacctd for seqno 1 (Critical Response)

After that, I cannot make any call again.
When I run “stop now” command, there is no response and terminal is hang.

my extension.conf,
exten => _00.,1,Dial(CAPI/ISDN1/${EXTEN:4},60,ftr)
exten => _00.,2,Hangup()

Is this right?
What is the problem that make my asterisk unstable at all?
Or how can I check for the unhandle call?

Thanks in advance.