With the 21.5.1 May release there is a new function that will allow a contact to be opted out of phone communications.
This will have the effect of disabling the Click to Call functionality from the Lead detail screen by prompted the user that the Contact has opted out and to try an email or SMS.
This will also cause the Phone number to be hidden in both the Contact and Lead Detail screen. To uncover the phone number, simply click the grey area to reveal it.