Before giving it a factory reset contact apple for support. If you have purchased your 6/+/s model within the past ninety days you are still within full complementary technical support offered with apple care +. Visit apple.com/support to get live chat (connected within two minutes), telephone support or to schedule an appointment with an apple expert to get your device running the way you intended.
Usually it is something simple, so contacting Apple's team of trained professional experts has a 99% success rate of correcting any issue during the first interaction.
This fixed my Visual Voicemail issue! After many calls into tech support they had to escalate the issue. I found your post and BAM completely fixed. Thank you for saving me from a reset!