roger@roger-desktop:~$ echo '+++'>/dev/ttyS4
echo ATM2L3>/dev/ttyS4
echo ATD>/dev/ttyS4
(no result at all, no sound, totally blank line)

More modem info
http://www.usr.com/support/product-template.asp?prod=5610c
http://www.usr.com/support/5610c/5610c-ug/index.html  >REFERENCE >select your choice of command sets, S-registers, etc.

All I could see in dmesg, can't guarantee I didn't miss anything :
[    1.176994] Serial: 8250/16550 driver, 32 ports, IRQ sharing enabled
[    1.360190] serial 0000:04:01.0: PCI INT A -> GSI 17 (level, low) -> IRQ 17
[    1.381155] 0000:04:01.0: ttyS4 at I/O 0xd000 (irq = 17) is a 16550A

roger@roger-desktop:~$ lspci
00:00.0 Host bridge: Intel Corporation Ivy Bridge DRAM Controller (rev 09)
00:01.0 PCI bridge: Intel Corporation Ivy Bridge PCI Express Root Port (rev 09)
00:14.0 USB controller: Intel Corporation Panther Point USB xHCI Host Controller (rev 04)
00:16.0 Communication controller: Intel Corporation Panther Point MEI Controller #1 (rev 04)
00:19.0 Ethernet controller: Intel Corporation 82579V Gigabit Network Connection (rev 04)
00:1a.0 USB controller: Intel Corporation Panther Point USB Enhanced Host Controller #2 (rev 04)
00:1b.0 Audio device: Intel Corporation Panther Point High Definition Audio Controller (rev 04)
00:1c.0 PCI bridge: Intel Corporation Panther Point PCI Express Root Port 1 (rev c4)
00:1c.2 PCI bridge: Intel Corporation Panther Point PCI Express Root Port 3 (rev c4)
00:1c.4 PCI bridge: Intel Corporation Panther Point PCI Express Root Port 5 (rev c4)
00:1c.7 PCI bridge: Intel Corporation Panther Point PCI Express Root Port 8 (rev c4)
00:1d.0 USB controller: Intel Corporation Panther Point USB Enhanced Host Controller #1 (rev 04)
00:1f.0 ISA bridge: Intel Corporation Panther Point LPC Controller (rev 04)
00:1f.2 SATA controller: Intel Corporation Panther Point 6 port SATA Controller [AHCI mode] (rev 04)
00:1f.3 SMBus: Intel Corporation Panther Point SMBus Controller (rev 04)
01:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI Device 683f
01:00.1 Audio device: Advanced Micro Devices [AMD] nee ATI Device aab0
03:00.0 PCI bridge: Integrated Technology Express, Inc. Device 8892 (rev 30)
04:01.0 Serial controller: 3Com Corp, Modem Division 56K FaxModem Model 5610 (rev 01)
04:02.0 FireWire (IEEE 1394): Texas Instruments TSB43AB22A IEEE-1394a-2000 Controller (PHY/Link) [iOHCI-Lynx]
05:00.0 USB controller: NEC Corporation uPD720200 USB 3.0 Host Controller (rev 04)
06:00.0 SATA controller: Marvell Technology Group Ltd. 88SE9172 SATA 6Gb/s Controller (rev 11)
roger@roger-desktop:~$ 

sudo lspci -vvvk   found the modem :
04:01.0 Serial controller: 3Com Corp, Modem Division 56K FaxModem Model 5610 (rev 01) (prog-if 02 [16550])
	Subsystem: 3Com Corp, Modem Division USR 56K Internal V92 FAX Modem (Model 5610)
	Control: I/O+ Mem- BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Interrupt: pin A routed to IRQ 17
	Region 0: I/O ports at d000 [size=8]
	Capabilities: [dc] Power Management version 2
		Flags: PMEClk- DSI- D1- D2+ AuxCurrent=0mA PME(D0+,D1-,D2+,D3hot+,D3cold+)
		Status: D0 NoSoftRst- PME-Enable- DSel=0 DScale=2 PME-
	Kernel driver in use: serial

roger@roger-desktop:~$ sudo setserial -av /dev/ttyS4
[sudo] password for roger: 
/dev/ttyS4, Line 4, UART: 16550A, Port: 0xd000, IRQ: 17
	Baud_base: 115200, close_delay: 50, divisor: 0
	closing_wait: none
	Flags: spd_normal skip_test
roger@roger-desktop:~$ 

roger@roger-desktop:~$ sudo stty -a -F /dev/ttyS4
[sudo] password for roger: 
speed 115200 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^A; eol = <undef>;
eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R;
werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread -clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff
-iuclc -ixany -imaxbel -iutf8
-opost -olcuc -ocrnl -onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt
-echoctl -echoke
roger@roger-desktop:~$ 

Here is vendor ID :
$ sudo lshw ...
 *-pci
                description: PCI bridge
                product: Integrated Technology Express, Inc.
                vendor: Integrated Technology Express, Inc.
                physical id: 0
                bus info: pci@0000:03:00.0
                version: 30
                width: 32 bits
                clock: 33MHz
                capabilities: pci pm subtractive_decode bus_master cap_list
                resources: ioport:d000(size=4096) memory:f7d00000-f7dfffff
              *-communication
                   description: Serial controller
                   product: 56K FaxModem Model 5610
                   vendor: 3Com Corp, Modem Division
                   physical id: 1
                   bus info: pci@0000:04:01.0
                   version: 01
                   width: 32 bits
                   clock: 33MHz
                   capabilities: pm 16550 cap_list
                   configuration: driver=serial latency=0
                   resources: irq:17 ioport:d000(size=8)

THANKS !