There arent that many options.

Internal modems end up being a problem where Lightning hits the line and frys your server.

http://www.lavalink.com/products/serial/prod_quattro_pci_3_v.html

http://www.racoindustries.com/lavaquatropci3.htm

http://www.stayonline.com/serial_cards/5857.asp

System Requirements:
1 PCI or PCI-X slot, 3.3 volt, 32 or 64 bit

Ports/Connectors:
4 nine-pin serial ports; two on-board, two on header cable

Its best to get DELL modems for DELL servers so that you can get DELL support.

http://accessories.us.dell.com/sna/productdetail.aspx?sku=A0059344&cs=19&c=us&l=en

http://www.nextag.com/serv/main/buyer/ProductM.jsp?kw=3Com_U_S_Robotics&ptitle=55148149&pdir=0&click=n&node=&core=&page=1000&lgnode=&lgsearch=SERIAL_MODEM

Early Hayes External Drivers
http://www.hayesmicro.com/Tech_CS/legacy.htm