What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
By setting the field to required, the form user will be force to provide answer.
And by judging your question, I think you must be also needing to force them to use 'cellphone format" number not the telephone format number
You can do that by doing this.
1. Set the Input Mask to ON
2. Provide the Mask Value for cellphone number format you want. i.e. (####) (######) to have (0917) (1234567)
Please inform us if you need further help.