Email subscription pages can pre-populate the Email field with a contact's stored email address to show their email subscription preferences.
Similarly SMS subscription pages can also pre-populate the Email field but it would really be useful if the Phone field could also be pre-populated with the contact's Mobile Number and show their selected SMS subscription preferences.
Leaving the Phone field as a free textbox means that contacts can enter their number in a different format than we have stored in their contact record. They might enter a different number all together even.
If an SMS subscription page is submitted with a different Mobile Number or the stored number in a different format, this won't associate the contact with the chosen SMS subscription preferences and so they could either not receive SMS communications they have subscribed to but, more seriously, they could receive SMS communications they have unsubscribed from.