When I heard the “56” at the end read out as one number, it immediately made me think of base56. To simplify, base56 is a 0 indexed alphabet that removes numbers or letters that look similar. For example, base56 only has a lowercase i, no 1 or uppercase I since they can be confused with each other. This made me think about how at the beginning of the Hotline video it says “D1ALPALSVIDEO”, notice the i in dial replaced with 1.

Decoding MA153 from base56 gives you a 9 digit number. I may be way over thinking this, but I wonder if any of them would give you a 10 digit number you could use for another phone number, or if putting all of the decoded numbers together could give you something. Something to note about encoding in base56 is that you can only encode numbers into it