Think your cell number last digit as X and your age as Y then
(X*2+5)*50+Y+365-615
=100X+250+Y-250
=100X+y
X can be 1,2..9
Any number multiply by 100 mean it come as first number with two zero
That's why you get "The last 2 digits as your age and The 1st digit as your cell's last number".
But this can be fail for two scenarios.
1) your age is equal or more than 100 years.
2) your cell number end with 0.