I just got my modem to work. It was on Com3 in Windows XP and got assigned to COM5 on Linux.
Here are some commands and responses that I got:

>>>>>>>>>lspci -v

02:02.0 Serial controller: US Robotics/3Com 56K FaxModem Model 5610 (rev 01) (prog-if 02 [16550])
Subsystem: US Robotics/3Com: Unknown device 00d7
Flags: medium devsel, IRQ 22
I/O ports at a400 [size=8]
Capabilities: [dc] Power Management version 2


>>>>>>>>>lspci -vv

02:02.0 Serial controller: US Robotics/3Com 56K FaxModem Model 5610 (rev 01) (prog-if 02 [16550])
Subsystem: US Robotics/3Com: Unknown device 00d7
Control: I/O+ Mem- BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
Status: Cap+ 66Mhz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
Interrupt: pin A routed to IRQ 22
Region 0: I/O ports at a400 [size=8]
Capabilities: [dc] Power Management version 2
Flags: PMEClk- DSI- D1- D2+ AuxCurrent=0mA PME(D0+,D1-,D2+,D3hot+,D3cold+)
Status: D0 PME-Enable- DSel=0 DScale=2 PME-

>>>>>>>>> cat /proc/pci
Bus 2, device 2, function 0:
Serial controller: US Robotics/3Com 56K FaxModem Model 5610 (rev 1).
IRQ 22.
I/O at 0xa400 [0xa407].

cat /var/log/messages |grep ttyS
Aug 8 18:23:13 lightning kernel: ttyS04 at port 0xa400 (irq = 22) is a 16550A
Aug 8 19:49:53 lightning kernel: ttyS00 at 0x03f8 (irq = 4) is a 16550A
Aug 8 19:49:53 lightning kernel: ttyS01 at 0x02f8 (irq = 3) is a 16550A
Aug 8 19:49:53 lightning kernel: ttyS04 at port 0xa400 (irq = 22) is a 16550A

cat /var/log/messages
Aug 8 18:23:13 lightning kernel: Serial driver version 5.05c (2001-07-0 with MANY_PORTS SHARE_IRQ SERIAL_PCI ISAPNP enabled
Aug 8 18:23:13 lightning kernel: ttyS00 at 0x03f8 (irq = 4) is a 16550A
Aug 8 18:23:13 lightning kernel: ttyS01 at 0x02f8 (irq = 3) is a 16550A
Aug 8 18:23:13 lightning kernel: Redundant entry in serial pci_table. Please send the output of
Aug 8 18:23:13 lightning kernel: lspci -vv, this message (12b9,1008,12b9,00d7)
Aug 8 18:23:13 lightning kernel: and the manufacturer and name of serial board or modem board
Aug 8 18:23:13 lightning kernel: to [email protected].
Aug 8 18:23:13 lightning kernel: ttyS04 at port 0xa400 (irq = 22) is a 16550A



after all that, I just did:
setserial /dev/ttyS4 irq 22 ( you will need to check your IRQ)
ln /dev/ttyS4 /dev/modem

Hope this helps.
Frank