For telephone numbers, I have taken a table calender, [remove all the sheets and use the four sides ]and made one page for commercials like Gas, Milk coupon, all banks, electrician, plumbers and other essential services numbers like BSNL , Broadband etc.


Next is friends numbers, then on the other side family where as per the importance the land line and mobile numbers are written.No guessing is needed this is placed next to the telephone apart from storing them in one's mobile phone.

Added to this is on a 'POST IT', my husband's, son's daughter's DIL, SIL and my mobile numbers are written and pasted on the dining door shelf , this will help in case of emergency for the friends or neighbours who come to our rescue.

My neighbours have all our phone numbers and a set of house key permanently with them.

PS

as you write the gas agency number write your consumer number also , nowadays its computerised tele services for booking a refill.