Send the following on WhatsApp
Continue to ChatQuery : You can send them sms (which is costly) and/or if user possesses an advanced mobile device you can send push notifications which are almost free but requires developing a phone app. For https://vmapp.org/query/88281/You-Can-Send-Them-Sms-Which-Is-Costly-And-Or-If-User-Possesses-An-Advanced-Mobile-Device-You-Can-Send-Push-Notifications-Which-Are-Almost-Free-But-Requires-Developing-A-Phone-App-For/