First I used Skype but then as I wanted to have something more flexible I
decided to try "proper" VOIP. My wife has relatives in Singapore and
Malaysia so after a bit of research I chose VoipDiscount as my outgoing call
provider. I chose (wrongly I now think) a Linksys PAP2T as my ATA as it was
cheap.

It has two ports and so I have a DECT phone for VOIP outgoing calls, a DECT
phone for incoming VOIP calls (very rarely gets used) and a DECT phone for
my normal BT line. I do not want to use my VOIP number for everyday incoming
calls and wish to stick with my BT line for that.

I do not want to change my router as I am very happy with its stability and
speed.

What I would like is to replace my ATA with one that allows my to plug my BT
land line into it as well as a network cable and will allow me to have
multiple providers on one phone so that all incoming calls (VOIP or BT) will
ring on the one phone and that via (easy to setup!) dialling plans calls
will be routed either over my BT line or by VoipDiscount account.

Any suggestions please?