Unfortunately, my phone doesn't seem to be ringing properly, which is a problem that I've had for some time. Calls go striaght to my voicemail; there's no sign of there being a missed call, just a message from 171 alerting me to a new voicemail message.
It happens to me as well. Calls are going to voicemail first. When someone rings me after the first attempt again then call will reach me no problem. But firstly it goes to my voicemail. When I was testing I get one dial tone and then voicemail is answering. I started to be happening about 2-3 weeks ago. I use iPhone 4s 6.1.1.