http://www.networkrail.co.uk/aspx/6538.aspx
says...

---quote---
Telephones

There are no public telephones at the station.

The nearest ones are over the road in the entrance to King's Cross St
Pancras Underground.
---/quote---


I appreciate that public payphones are rather 20th century, but I'm
still a tad surprised by this.

On a related note, are there any subterranean payphones (i.e. deep
platform level) on the London Underground network still in existence? If
not, when did the last one go?

(And apologies for the slightly laboured subject line!)