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.
3. Enter your Pin Number. You will need to know your pin or passcode in order to check your voicemail. If you don't have this code, you will have to retrieve it from your carrier. You will hear instructions to input your pin. Hit the pound key after you enter your pin. Follow instructions on how to listen to your voicemail. It's usually to push a certain number (like 1). That's all there is to it. You should be able to hear your voicemail messages now.

2. Try calling your voicemail directly. Open Phone → Make sure you are on Keypad → Press and hold the number 1 for a few seconds. It will call your voicemail; You can check your voicemails here.
You can tap on Record to start recording a new iPhone voicemail greeting and then Stop when you are done. Tap on Play to listen to it so you don’t have an embarrassing mistake or noise in it. When you are satisfied with the message, tap Save in the upper right.