I am going to organise a quest for teenagers.
I have a vintage phone like below:
I am looking for ideas how to use the rotary phone with numbers 1,2, ..., 9, 0 and letters A, B, C, F, H, I, K, L, M, R.
My attempt is: to use a "A1Z26" cipher (in my case A1R0), for instance, 3188 - CALL, 2188 - BALL, 619 - I AM, 4188 - FALL, 7688 - KILL, 2688 - BILL.
What is an alternative to the substitution cipher?