If my PC is switched off and I pick up my telephone, the PC boots up.
Have disconnected modem but it still happens.
Can you help?
W. Frankfurt

Try unplugging the PC.

you say you have disconnected the modem ? how did you disconnect it ?

most pc's have an option in the bios where you can set what will wake up the pc 
eg. action on COM2 or wake up on activity on the LAN
 check those settings

If you have disconnected the modem by unplugging it from the phone
jack, then the computer can't be getting the signal through the phone
line.  However if you have a cordless phone and wireless network,
there may be some interference if both are 2.4 GHz (theoretically, I
haven't looked to see if this problem has been reported).  Then the
computer may be set to wake on LAN as mentioned above, and be
interpreting the signal from the phone as a signal from another
computer trying to connect.

Even though the solution depends upon the type of your phone, i.e.
cordless or not, or frequency if cordless, and the networking devices
installed other than the modem on your computer; i.e. do you have
bluetooth, i.link, zigbee or whatever compatible device that might
have interference with your phone, usually each solution provided
might include a step that mentions about shutting down the "wake on"
property of the related device on your computer.

Mostly you can do so by editing your settings with BIOS(Basic Input
Output System). You can reach your computer's BIOS at boot time, after
powering the computer up. Most computers accept keystrokes on "DELETE"
and/or "F2" keys as signals top launch BIOS. If your computer wants
you to invoke BIOS in a different way, it will probably tell how in
the initial boot screen(probably as "Hit F2 to enter SETUP"). If this
is not the case you can try hitting the function keys(the keys F1-f12)
one by one.

After entering BIOS, depending on your BIOS you might face with
different interfaces, however usually there is some tab named "Boot"
or "Power Up". Within that tab try to locate all the "wake on" or
"power up" settings and shut them all(if you're specifically using one
feature, you might leave that one however). Afer doing appropriate
changes, save your changes & reboot your computer.

If your issues are not resolved still, you may consider to find a
driver software that is located "within" your own operating system
that might be controlling the "wake on" property. Since I have no
information on your hardware I can't comment on what to do; however if
you provide the hardware you have we can help I guess.