ISDN Modem

Hi,
I have a client who wants to install an ISDN modem on his server to allow RAS connections. The RAS users will be using in most cases POTS (analog PCMCIA modems connected to a normal telephone line). In the first instance is this possible ?
Distributors, I talked to are divided on this issue.
Does anyone know of an ISDN modem I could use to do this, if possible?