.... especially given your symptoms.
... and your com port has taken a walk.
You didn't say if your internal modem is an ISA with jumpers or a PCI PNP modem.
Depending, the specific resolution steps are different - but the core issue is the same. The modem is looking for a com port that is available to the OS so they can communicate with each other. (It is also possible to install some fax/modem software what will capture the com resources.)
REMEMBER that a com port is a combination of resources - and that a SERIAL port is a physical thing. So, you need to ....
1)look in device manager at the serial ports available, if any.
2)look in the bios to see if the serial ports have been allocated resources that the OS needs.
Sorry, can't be more specific - different OS, not to mention proprietary bios - all have unique answers. But the core issue is always the resources assigned to the com ports.
Good luck!!