If a business promotes calling on WhatsApp to collect customer numbers, does this count as an opt-in?

Yes, but with a caveat—the business must still create a flow (such as a form on their website) that complies with  WhatsApp's opt-in rules. The business must make clear to the customer that if they do call the number, they are actively opting in to receiving a specific type of message via WhatsApp.

Opt-ins must be collected before you initiate a conversation with a customer using WhatsApp. Also, businesses must clearly state that a person is opting in to receive messages from the business over WhatsApp. It should be clearly stated as well the business’ name that a person is opting in to receive messages from.

it’s the company's responsibility to store customer opt-ins and to ensure each customer you choose to contact has already agreed to receive messages from you on WhatsApp.

Find more information about how to implement opt-ins here.