I believe an existing relationship qualifies as you describe it, but that isn't the approval you must have for this. I've read that that with your customers consent you could converse with then via text and provide information and transactional messages (you have an appointment, there was a large withdrawal). But marketing messages require separate written approval to text them and you need to offer an opt-out. Your written authorization could be electronic the same as "text STOP to have these messages stop being sent" could be. Different uses of the text require different levels of approval from your customer.
_________________________
AndyZ CRCM
My opinions are not necessarily my employers.
R+R-R=R+R
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell