746 not ringing and no dial tone
 

Just a bit of a shot in the dark: a friend of mine, who doesn't live locally, has an old 746 ( at least I'm 95% sure it's a 746 ). It's been in use previously but he moved recently and it hasn't worked since. If he calls the number from his mobile, he hears a ringing tone but the phone itself doesn't ring; also, there's no dialling tone ( or sound of any kind ) when he picks up the receiver. I've told him to try a different phone in the same socket; the different phone worked fine. I've told him to try the same phone plugged into the internal master socket; no change.
I know that 'remote diagnosis' or indeed 'diagnosis by proxy' ( ! ) is tricky at best, but does anyone have any clues as to what direction to look in? For what it's worth, my thought was that if there's no dialling tone and the phone's not ringing, the problem might be either with the plug that goes into the wall socket, or the connections from that plug to the terminal block inside the phone.
Any inspiration, anyone?

Re: 746 not ringing and no dial tone
 

I think you're right that the first port of call is confirming electrical continuity from the plug to the internal connections.

Assuming that the 'phone has been wired for third-wire ringing from an external capacitor, the lack of both ringing and dial tone suggests to me a problem with the "A" leg (which is common to both).

Maybe a squirt of contact cleaner on the plug contacts would be worth a try before anything else, just to eliminate the possibility of oxidised contacts causing the problem.