How to add phone number field to simplereservation/add page
I think if you don't want to edit module, you may use comment field for telephone number. It's much easier to customize comment label and change it to phone number, than to edit module's code for additional field to store in database and display in form. Anyway it will be your preference.

UPD: tried to do something. If you don't need user comment, you can change Comment label, make it required field, and change description. (better solution is to try alter form)

Result: 

Where to replace code in sites/all/modules/simple_reservation/simplereservation.module:



You just can compare lines from original and my example.