If the problem is with your phone, the factory reset has a good chance of solving it. If not, contact your mobile service provider, to carry out some troubleshooting to help resolve the problem from their end.
I like to use a “three strikes and you’re out” rule. Meaning, after the 3rd attempt, if I haven’t heard back from them, I take them out of my current call list (I move them back into my drip marketing campaign and let that do its job)

This article will introduce you how to change voicemail messages/greetings and voicemail number on iPhone 7/SE/6s/6/5s/5c/5/4s/4/3GS. Read below and give it a try. Of course, you can change the voicemail on your iPhone and set it to whatever you want. Your carrier will give you a voicemail box, which has a standard message. You can change the message into anything that you want. You can use your name, and leave a catchy message, and you can even sing a song if you want to. You have a wide range of options when changing your iPhone voicemail settings; you cannot only change the type of message, but also how to retrieve them, when you should be prompted, and a lot more. Part 1. How to Change and Record Voicemail Greetings on iPhone Part 2. How to Change the Voicemail Number on iPhone Part 3. Can I Recover A Deleted Voicemail on My iPhone Setting a voicemail greeting is important when we missed a call and let the people calling you know they have reached out to the right person and remind them to leave a message after the greeting. To record a personalized voicemail greeting, here are the steps that you have to follow. This action will lead you to the voicemail menu on the iPhone, and it will dial the voicemail service offered by your carrier. This is dependent on the carrier and also the plan that you have. You will have to follow the initial process given by the voice prompts from your carrier. This will allow you to record your greeting. If your carrier plan allows visual voicemail, you will see a "Set Up Now" button, which you should tap. Depending on your carrier, you will be asked for a password for your voicemail. Make sure that it is one that you can easily remember. Each carrier has a certain number of digits. You will be asked to repeat the password for it to become effective. Using visual voicemail, you can decide whether to use the "default" greeting or create a "custom" one. When creating a custom greeting, you will be allowed to review it before you confirm the recording. Simply click on the record button, and then click stop when you are done. Once you have reviewed it, you can click on "Save" to set it as your greeting. After you set your personal voicemail greeting, your iPhone dials a number to recover or access your voicemail. In this way, you can change the 9-digit voicemail number by following easy steps: On the keypad, type "*5005*86*0123456789#" – NOTE: "0123456789" is the number to which you want your voicemails to be sent to. Once you have entered the above number, tap the "Call" button, and the new number will be set. All your voicemails will be sent to the new number. That's it. As you can see, it is so easy to change voicemail number on the iPhone. Yes, it's possible to get your deleted voicemail back; losing an important voicemail is bad, but fortunately, we provide 2 possible ways for you to retrieve them back. Let's check them out! Step 2 Scroll down at the bottom of the page, you can see "Delete Messages", please tap on it. Step 3 Find the voicemail that you don't want to delete and tap "Undelete" to retrieve it back. For iPhone users, we recommend you to try dr.fone - Data Recovery to recover deleted messages and voicemail. Here we listed some key features to tell you why you should choose it. It supports all iOS devices: This means that you can use it to recover voicemail on iPhone, iPad, etc. Handy Preview feature: With this feature, you get a preview of the voicemails before recovering them. Full Data compatibility: This tool can access up to 18 different types of data. This makes it very versatile for the recovery of images, video, voicemail audio, and a lot more. Easy to use: The user interface is easy to follow, that even a novice will be able to handle it with ease. Allows for partial recovery: You can select the data that you want to recover. If you only want certain voicemail messages, you only select them and leave the rest. Download and install dr.fone on your computer, and also connect your device to with PC. There are three options list on the next screen; it's you choose one of them to scan the device, you can also choose file types to start scanning. Once you finished scanning, all the data that you deleted have been displayed on the screen. Find the voicemail that you would like to recover. After reviewing the deleted files' results, you can simply recover these files anywhere in your local directory. In this article, you have cleared how to record and change the voicemail number; and provide some solutions to recover deleted voicemail back. I hope you have already recovered your voicemail easily without any further loss. Hot Articles The Best Way to Recover Snapchat Messages on iPhone
5. US Cellular: Dial *86 (*VM) from your US Cellular device, then press “Call.” This command will dial you into the voicemail system’s main menu. Press “3” to access the Greetings menu.