(1) no internal modem installed
$ dmesg | grep ttyS

[    4.355605] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    4.356167] serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A
[    4.358402] 00:02: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    4.359399] 00:03: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A

These are the two onboard serial ports.

$ cat /proc/interrupts
           CPU0       
  0:      24900    XT-PIC-XT        timer
  1:        203    XT-PIC-XT        i8042
  2:          0    XT-PIC-XT        cascade
  5:          1    XT-PIC-XT        SoundBlaster        *
  6:          3    XT-PIC-XT        floppy
  7:          1    XT-PIC-XT        parport0
  8:          0    XT-PIC-XT        rtc0
 10:          0    XT-PIC-XT        uhci_hcd:usb1
 11:         80    XT-PIC-XT        eth3
 14:       1404    XT-PIC-XT        ide0
 15:          0    XT-PIC-XT        ide1

An external modem in use attached to serial port 1 or 2 would
show up as interrupt 4 or 3 respectively.

(2) internal USR ISA jumpered to PNP
$ dmesg | grep ttyS

[    4.409325] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    4.409909] serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A
[    4.412109] 00:02: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    4.413212] 00:03: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A
[    4.417751] 01:02.00: ttyS2 at I/O 0x3e8 (irq = 5) is a 16550A  *

* Modem works, but this conflicts with ISA Soundblaster card in the
  machine which uses irq 5, so there is no sound. 

$ cat /proc/interrupts
           CPU0       
  0:      97779    XT-PIC-XT        timer
  1:        718    XT-PIC-XT        i8042
  2:          0    XT-PIC-XT        cascade
  5:       4770    XT-PIC-XT        serial        [modem in use]
  6:          3    XT-PIC-XT        floppy
  7:          1    XT-PIC-XT        parport0
  8:          0    XT-PIC-XT        rtc0
 10:          0    XT-PIC-XT        uhci_hcd:usb1
 11:         86    XT-PIC-XT      
 14:       1886    XT-PIC-XT        ide0
 15:          0    XT-PIC-XT        ide1

(3) internal USR ISA jumpered to com3 irq 4
$ dmesg | grep ttyS

[    4.355656] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    4.356252] serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A
[    4.356800] serial8250: ttyS2 at I/O 0x3e8 (irq = 4) is a 16550A **
[    4.358627] 00:02: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    4.359622] 00:03: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A

** Both modem and sound work.

$ cat /proc/interrupts
           CPU0       
  0:     336830    XT-PIC-XT        timer
  1:        421    XT-PIC-XT        i8042
  2:          0    XT-PIC-XT        cascade
  4:        238    XT-PIC-XT        serial         [modem in use]
  5:          0    XT-PIC-XT        SoundBlaster
  6:          3    XT-PIC-XT        floppy
  7:          1    XT-PIC-XT        parport0
  8:          0    XT-PIC-XT        rtc0
 10:          0    XT-PIC-XT        uhci_hcd:usb1
 11:        357    XT-PIC-XT        eth3
 14:       1673    XT-PIC-XT        ide0
 15:          0    XT-PIC-XT        ide1

 Note that interrupt 3, i.e. the COM2 = ttyS1 is not used and is available 
 for a serial mouse, serial console (null modem), etc.