Hi,
If you are able to connect using a dial up, then your client and Pix configs are good.
If you are not able to make a connection through your ISDN link, then it is your PAT that is the issue. As of today, IPSec Through NAT option is not supported on the pix and will be supported in Pix Version 6.3(Ipsec Over UDP).
You will be able to make a connection through PAT because phase 1 uses UDP Port 500 but when it comes to encrypting traffic, ESP is a protocol and does not support PAT.
Regards,
Arul