We need to debug at isdn q931 along with the ppp level. By the way here is the url which helps you for configure and troubleshooting
http://www.cisco.com/warp/public/793/access_dial/5300.html
In that url pl. make sure that you configure the isdn switch-type for dms500 as dms100
isdn switch-type primary-dms100
If the things still dosen't work then we need to take a look at following debug with sh commands
debug isdn q931
debug dialer
debug ppp authentication
debug ppp negotiation
term mon
with
sh isdn status
sh run
Tejal