Recently a client needed a function or two for Gravity Forms to process UK telephone numbers, and asked that they are between 7 and 15 characters in length. The following Gravity forms filters do exactly what’s needed.

Paste them in to your theme’s function.php and with a little modification you should be good to go.

UK Phone Number Format

Using a simple regex function, we can create a new mask for telephone fields that eliminates unnecessary characters for validation and return the default error if it's incorrect, like so:
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That function adds an additional option (UK Telephone Number) to the Phone Format box when editing the Gravity Form field as shown below.

Limit Between a Certain Number of digits

Not the most elegant of functions, but working well all the same, the function below sets a minimum and maximum number of characters for a Gravity Form field, and displays a different error message for each. The part of the function that says 'gform_field_validation_1_5' indicates the location/ID of the field (Form 1, Input Field 5).

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