While holding the telephone receiver to my ears my fingers felt numb & tingling sensation . why?

Number of things: This could be a number of things, depending on some other factors within your history. I would be concerned potentially with something like carpal tunnel syndrome or other neuropathy. Suggest seeing your doctor with a potential need for further investigations if it persists.

Nerve Irritation: Since the activity of holding the telephone could put pressure on nerves at the wrists or elbow. That could cause irritation of the ulnar nerve which could cause numbness of the 4th and 5th fingers. At the wrist the median nerve irritation causing numbness of the first 4 fingers. Change hands when holding the phone or use a speaker phone to rest the nerves. If symptoms continue see a physician