I would like to be able to place a call using a WhatsApp number from my iOS app. How can I do that?
You should ask WhatsApp how to do so. The solution lies in URL Schema: https://www.whatsapp.com/faq/de/iphone/23559013
Here the code copied from their site, in Objective-C. I'll keep it as an exercise to do so in Swift:
NSURL *whatsappURL = [NSURL URLWithString:@"whatsapp://send?text=Hello%2C%20World!"];
if ([[UIApplication sharedApplication] canOpenURL: whatsappURL]) {
[[UIApplication sharedApplication] openURL: whatsappURL];
}
And just to make it sure: They won't let you send messages directly to someone, and even more: They won't let you send you messages silently, without WhatsApp being opened before.