If I'd followed Backwoodsman's advice, I probably would have missed a much needed hospital appointment! The caller first called my mobile phone, but I only discovered the missed call much later - no message left, and from an unknown number. She did then call the house phone, and we did pick up the phone - appointment given for 4 days later, "but we'll confirm by letter" - the letter finally arrived on the day of the appointment about an hour before I needed to leave home! (The envelope had been rubber stamped "First Class" but the postmark showed it had been sent 2nd class!)
I am very glad I DID pick up the phone that time, as been waiting for that appointment for months!