The question regarding the ticks has been answered in the first link.
With regards to the phone ringing - if you ring their phone on a normal call and it goes to answerphone, it could mean a few things; such as the other person is on a call and their settings are such that an incoming call diverts to answer phone .... or it could just be that their phone doesn't have a signal. Whatsapp uses the internet to make phone calls, so you could in theory have no phone signal but still have an internet signal (maybe connected to WiFi??), so you'd be unable to connect to them using a "normal" phone call; but they'd still be accessible by making a Whatsapp call.
Hope this makes sense :)