Circuit/device to make internal phones ring?

[LeeUKHA <mailinglists@xxxxxxxxxxx>]

Last week I missed a delivery (of new toys) by being the shower when the
CityLink van called by.
Annoyingly I thought I heard the door bell, so he didn't just stick the
card in and run...

Anyhow, it struck me that now I have a shiny new VIOM, I could hook it
up to the doorbell and then get the HA server to ring the internal
phones just once, rather like the Comfort system...

Despite a good deal of Googling, all I can find are circuit designs to
make a phone ring, as long as it's NOT actually plugged into a phone
line. Fair enough as who knows what may happen at the BT exchange by
putting ringing into the external phone line, but as I've got a PBX
installed, is that actually a problem? The only problem then is how to
attach the circuit?

Anyone?

Lee