Web Form Formula
There is a option in edit form options - Web Form Formula.
Add 15 to customfield(18230) whenever the user fills current web form.
update customfield 18231 with a question, and variable name "cellphone" (variable name must be assigned to a question).
if question type is a combo box like "1=Yes;2=No"
@form.variable.text@ will be replaced with Yes or NO
@form.variable@ or @form.variable.value@ will be replaced with 1 or 2