Thanks Thom
My phone is Android so I don't actually have any "Mobile Orbiters" to get it to ring. I though I might be able to get around it by putting an extension into the "Other Notifications" section of the "Cellphone Notifications" page, but I still didn't get any Doorbell events firing when it initiated a call to that extension.
I also just worked out that I can set the EK_AlertType on my door camera to 6, which makes camera movement my doorbell, and I can then configure it in the "Active Sensors", but sadly I have a bit too much background movement for it to be useful.
So I decided to just get a GC-100 to set up a doorbell properly.
Cheers
Phil