Re: You mean, like, Caller ID?
"Unless you present to me a known CLI that I have chosen to accept in the past, nothing gets through"
I agree with most of what you say there, whitelisting is the way to go although I was glad I did answer a withheld number a few months ago, it was my Doctors with a rather important and potentially lifesaving message for me.
I raised their lack of presentation numbering at my subsequent visit and was assured they would pass it on to the 'technical people'. I requested that they contact me for an explanation if they needed as I had more than a passing few years in corporate comms - both 2 wire and SIP and would like to understand their rationale towards withholding the number altogether - but heard nothing more.
I've got a feeling they are part of a larger hosted platform with nothing more than a bit of QoS'd pipe to the far end and that may well be a BT Avaya platform as the whole surgery is adorned with the rather cute elephant's foot Avaya 46xx series. OTOH they probably have a ropey old IP Office at the bottom of the mop cupboard and a now long-gone maintainer who saw them coming and flogged off all the old crap he had knocking about!