To activate Visual Voicemail, dial 1217 from your iPhone and a recorded message will confirm it has been switched on. When a message is left in your voicemail mailbox, you'll see a notification against the ‘Phone’ widget on your iPhone. When you activate Visual Voicemail, your current personal greeting will carry over.
When voicemail is not working on your iPhone, a lot of people assume that there’s a problem with their wireless carrier, so they immediately call Verizon, AT&T, T-Mobile, or another carrier’s customer support hotline. However, a lot of the time the problem is really caused by a software issue on the iPhone itself. Your iPhone isn’t downloading voicemails from your wireless carrier; or The Phone app on your iPhone isn’t working correctly

It may take up to 5-10 minutes for the phone to reconnect to the network and register itself.
Android phones can access voicemail by holding down the 1 key on the dial pad until the phone dials your 10-digit phone number. You will automatically connect to your mailbox and be prompted to enter your temporary password; this password is the last 4 digits of your phone number followed by the # key. Start the Google Voice app. Tap the three horizontal lines at the top left of the screen (sometimes called a hamburger menu), and then tap “Settings.” In the Voicemail section, tap “Voicemail greeting.” Tap “Record a greeting.” How do I access my voicemail? To call your voicemail on an Android phone, simply open your phone’s dial pad and hold your finger down on the “1” key. You can also call your voicemail from a different phone by calling your own number and tapping the pound key. How do you check your voicemail?