I don’t use the phone much, but if someone tells me a phone number, I chunk it 3-3-2-2 or 3-2-2 (if it’s in my area code). Each image has a one-syllable pronunciation, so if I only need to hold it in memory for a minute I can just store it as a three or four syllable nonsense word… otherwise link the images and place them somewhere.
There’s some more discussion of phone numbers over here by a guy who memorized a phone book: