WhatsApp's voice calling doesn't seem like it has changed a lot since it was first tested as an invite-only feature then released to everyone in March of 2015. But the service, from my personal experience, has become more reliable, less spotty, with fewer delays and disconnections. Around me, everyone uses WhatsApp calls, because everyone is on WhatsApp to begin with, but also because local calls still aren't unlimited or cheap on any plan, not to mention the exorbitant prices of international calls. WhatsApp calls have almost completely replaced regular calls for us, and it's not hard to imagine that it must be the same in other countries where WhatsApp is popular too. Just today, I was chatting with a drug company rep who told me that most if not all of her business calls (boss, clients, suppliers, etc) are done on WhatsApp now.
It's easy to spot the trends around you from every regular Joe on the street and realize that a service has taken off like wildfire. And wildfire it is. WhatsApp just announced that over 100 Million voice calls are placed each day. That's about 1100 calls every second. There's no mention of how many minutes that amounts to, but I'd expect it to be at least 5 minutes on average, based on anecdotal evidence from my surroundings. I've been on calls that lasted a few seconds and others that spanned several hours without interruption.
If you live in the US and you're having trouble imagining why WhatsApp has basically disrupted the entire VOIP market in a year, imagine a user base of over 1 billion persons, many millions of which live in countries where data is cheaper than calls or messages, and who just need your ordinary phone number to reach you through WhatsApp. No username, no email, no nothing. Facebook Messenger is the only real competitor with the same advantages that I can think of right now.
Oh and by the way, WhatsApp says it's committed to making calls better in the months to come — that might be an allusion to the upcoming video calls.