The new phone # database introduced in the 18.07 release includes both a Mobile phone checkbox as well as a Can sent text messages to this number checkbox. There’s also a place to enter free-form comments on each individual phone number. Also remember to take advantage of the existing marketing opt in/opt out flags. You could even use all of these elements in tandem with a special Tracker Type/Memo Type combination that specifically records a member’s written consent.
 
Another idea would be to create special phone number labels to be used when a member gives consent (or expressly forbids it, since that’s likely to be less common). Keep in mind, however, that the since the purpose of the labels is to record the member’s description of the contact number, this should not be your only indicator. However, it might still work in tandem with the comments and the other checkboxes to give you a more complete picture of the member’s wishes.