tel field in contact form 7 accepting non numerics

Using contact form 7, I have created this form – https://www.cloudsteer.com/contact-us/ using the following code:

<label class=”half”> First Name (required)
[text* FirstName] </label><label class=”half”> Last Name (required)
[text* LastName] </label>
<label class=”half”>Your Email (required)
[email* Email] </label> <label class=”half”> Mobile No. (required)
[tel* Mobile-Phone minlength:10 maxlength:12] </label> <label> Message (required)
[text* Description] </label>
[hidden Lead-Source default:”Contact Us Form”]
[submit “Send”]

Issue is with ‘[tel* Mobile-Phone minlength:10 maxlength:12] ‘ field which is accepting any value (numerla, non numeral, mix). Being a ‘tel’ field, should it accept only numbers? How can I fix this? (please don’t refer a plugin). Solution by editing internal file (formatting.php/functions.php etc.) would be highly appreciated.

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