Can I use the phone numbers of my Zomato customers to send offers on WhatsApp?

Usually no. Zomato's terms let you use customer data to fulfil the order, not to add it to a list or send unsolicited marketing. People who are already your customers, or who opt in, are different.

• Zomato's terms say customer data is for fulfilling the order, not for reselling or unsolicited marketing or feedback messages.
• The limits do not apply to people who were already your customers, or who give you their data directly and clearly opt in.
• WhatsApp also requires opt-in before you message people. Methods include a website, in person, on paper, SMS or by phone.
• The easy route is to ask for consent when customers order directly from you, in person or on your own ordering link.