Dail-in problem with serial ISDN Modem

[Henrik N <poxx_(_at_)_home_(_dot_)_se>]

Hi.
I'm having trouble to get my serial isdn modem to accept 
any incoming connections.
Dialing out is however no problem.
I tried the configuration examples in the man ppp and in 
the ppp.conf.sample file and I have used the ppp.conf file 
in the i4b package. 
I don't think i4b package would work because i have a 
serial isdn modem, as i recall the i4b package
is only for internal isdn cards. The only way i can get the 
modem to answer is to go to "term mode" in ppp interactive 
and manually type "ATA",the modem tries to authenticate but 
after that the modem disconnects. 
I would be glad if someone could give me some directions to 
solve this problem or if anyone can give me a sample 
ppp.conf file, or any other file that may be involved in 
recieving incoming connections on a serial isdn modem.

/Henrik