Official comment
The iPhone thinks the telephone number is a link and renders it with the default blue link color. You can choose either a different background color or add the following code to the Settings > Custom Header Code section of the form or page: <meta name="format-detection" content="telephone=no" />
That will prevent the device from changing the phone number into a link with the default blue color.
Android phones do not automatically change a phone number into a link, so this applies only to iPhones.
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