If your computer is booting up on its own, your phone line and your motherboard are conspiring to start the computer.

The BIOS on the motherboard may be configured to turn on the computer when it hears a phone signal arriving.

If you don't want the computer to start in response to an incoming call, change the BIOS settings in the computer's Setup program.

Look through Setup's various sections for the one where boot-operations options are listed.

There you should find a line reading something like On Modem Ring. Next to it you will see an option titled, Power On. Change the option to Stay Off, or some equivalent.