Data Protection:- the key is this:-
If they have given you the information personally (eg phone numbers) then you can certainly keep it and use it for the reason you gave them.
You cannot, without their expressed permission pass it on to any other person outside your firm or use it for other purposes aside from that which was explained at the time. You have a duty of care to maintain confidentiality too.
So if you say to your customer:-
"May I have your phone number so that I can contact you in relation to our window cleaning arrangements?" then you may use it for that and that alone.
That means anything to do with your window cleaning service - things such as reminding them about money outstanding, telling them you'll be around so can they leave their gate open, holiday arrangements, new methods of working etc.