India's telephone numbers are governed by the 2003 National Numbering Plan. The country code "91" was assigned by ITU in the 1960s. Total phone numbers consist of 10 digits, with area codes and numbers
100 is the square of 10 and the sum of first nine prime numbers. It is a semiperfect number with geometric mean of divisors 10. Has reduced totient 20 and Euler totient 40. Sum of cubes of first four positive integers
Belgian numbers consist of 9-10 digits for landlines and 10 digits for mobile. All national calls require leading '0' trunk code. Special services use 3-4 digits without area or trunk codes
ITU's Recommendation E.123 specifies national telephone number formats. Numbers use digits only when same, letters for groups. X character represents any digit in lists
Landline numbers consist of area code followed by local number. Major cities use 2-digit area code followed by 8-digit local number. Other areas use 3-digit area code followed by 7-8 digit local number. Calls within same area don't require area code
Landlines use area codes, mobile phones don't. Domestic calls start with "0" for long distance, omit for international. Large cities have 8 digits, other areas 7 digits