If they are making the call to the customer, then they are doing so on a number the customer has provided, therefore it should be accepted that if you ring that number you will either get the customer, or you will get someone associated (family/friend etc) with the customer, who will pass the phone over to them.
I used to work in a fairly large call centre, where the management were very strict on DPA checks with the customers. So much so that it used to result in many escalated calls to the managers. However, the rules only ever applied to inbound calls. We even left messages on voicemails and answer-phones with information relating to the account.