Network query

Hello,
i have a cisco 1800 series router with ISDN connection. last night it was workng fine. but from today morning. it can not connect to my remote office. although Link light is blinkng on router. and NT1 box.

i check the router configuration and give command sh isdn status

it is ok becoz layer 1 status is and says
ACTIVE
Layer says
Multiple Frame Established

After troubleshooting i found that When ever I give command
ISDN Test Call Int BRI0 phoneno

link light goes off in router and NT1 box

and ISDN status deactivated

any body help plz
becoz ISP is saying that From there end it is ok.

I m new in Network. anyone plz help help would be appreciated.


Regards
Rajesh
9331006431