Take the last number that called you or you called yesterday (or earlier if you did not speak on the phone yesterday). Example 0401234567 or +358-401234567.

Delete the first zero (or the land code). Example 401234567.

This number, (0) 40-12-34-56-7, gives:
=> 40 push ups
=> 12 air squats
=> 34 sit-ups
=> 56 back extensions
=> 7 burpees

i.e. the first two digits dictate the amount of push-ups you make.

The third and fourth digit dictate the amount of air squats you make.

The fifth and sixth digit dictate the amount of sit-ups you make.

The seventh and eith digit dictate the amount of back extensions you make.

The nineth digit dictate the amount of burpees you make.

If the phone number does not contain enough digits add 5s at the end. Good luck!