You haven't asked a specific question, so I don't know how long the telephone number is.
The way to find out how many numbers there are where the last digit is less than 7 is to do this multiplication:
7*10*10*...*10 for as many digits as there are or 7 followed by a number of zeroes. This number of zeroes will be 1 less than the length of the phone number, so if it's a 7-digit number we have 7000000 or 7 million numbers.
If not all the digits can be used in any other position then we multiply the number of available digits by 7 and then add the zeroes. For example, if the first digit can't be zero, then we only have 9 left to choose from so there will be 7*9=63 followed by zeroes, two fewer zeroes than the number length. For a 7-digit number there would be 6300000 possible numbers.