You have to use at least one location-spot to memorise this phone number. Else you’d have to memorise it in the cloud and soon forget the order of the numbers.
You would usually use a memory palace with a few spots on the location to memorise nine digit long number.
However, you can just use the link-method to connect the next digits to each other on a single location-spot. Doing this, you can than memorise more than nine digits on a single location as long as you link the next digits with the previous digits continuously.
Depending on what number system you are using, you can also use the person’s body to make action-links on and around the person. So, it would be on the left of the person, in front (or on top) of person, and on the right of the person. You can go even further with it, and use the hair, eyes, nose, etc as the pegs, so you can attach even more information.
To memorise numbers, you need a number system such as Major or Dominic or other.
To memorise a Phone number, you need to use a Phone Number Method such as the Method of Loci, Body Method, Words Method, other etc.
For example; If you want to memorise history dates, you’d need to again use the number system, and a method to memorise dates, history etc.
You’d have to of course Choose what method/s is most suitable for you for whatever you are trying to commit to memory at what situation you may be in, and use just that rather than learning many other methods. Use what works for you and stick to that to have your own system as a whole.