Strings of numbers are generally difficult to recall. Fortunately, most people now store phone numbers on their mobile phones so there is no need to recall them. However, that means that we have fallen out of the habit of recalling numbers.
It is a good habit to practise memorising sequences of numbers. If you lose your phone it might just save your life! We are more likely to remember numbers if they can be made meaningful in some way. Meaningful numbers might be, for example, your birthday or those of your friends and family, perhaps the number of your house or the year your favourite team won the FA Cup. If you can incorporate these meaningful numbers into the sequence you want to recall, that will make it easier.
For other numbers that don’t have any obvious meaning, you can link numbers to images. Do this by converting a number to letters. 23 would be BC (B being the second letter of the alphabet, C being the third). This might make you picture Bill Clinton. You are more likely to remember the image of a person you know or a famous person than a number. You can do the same for all numbers 00 - 99 and then link these images together to remember sequences of numbers.