If a phone rings once and then goes to voicemail or rings only briefly, it usually means either that your call is blocked or that the phone is not receiving calls at all. If the phone rings five times and then voicemail picks up, it's more likely that the person on the other end simply isn't picking up the call.
My iPhone rings for 15 seconds – which is actually eight rings – before it goes to voicemail, but even that’s not long enough if I’ve put the phone down in another room or it’s zipped into a pocket in my jacket.

If you’re calling from your own phone, you’ll be able to access your voicemail without entering a PIN. This will happen even if you’ve previously set one up.
If your carrier supports the iPhone voicemail feature, you can retrieve and listen to your voicemail right from within the “Phone” app and call your contacts back by simply tapping their messages. That type of functionality is pretty neat and useful, because it saves you actual time. If not supported by your carrier, you will not be able to see the visual overview of your voicemail inbox and use any of the related features. Tapping the “Voicemail” tab in your “Phone” app will simply call the number associated with your voicemail box. So let’s get going with creating our custom voicemail greeting on iPhone! How to record your own Voicemail on iPhone Phone → Voicemail → Greeting → Custom → Record → Save