Plenty have been wondering whats next for WhatsApp after the popular messaging service was purchased by Facebook for $16 billion last week, and now we have the answer: voice calls. According to TechCrunch, WhatsApp announced during a Mobile World Congress event today that it would be adding voice services to iOS and Android during the second quarter of the year. Thats 15 million more monthly users than Facebook detailed just last week when reporting the purchase. WhatsApp has already risen to an impressive popularity on basic messaging features alone, and the addition of voice calls should only enhance that further when they begin to roll out later this year. Its likely that voice services will fall under this too, while breaking it out as a second offering could even provide an additional revenue stream.