The default voicemail greeting on the iPhone plays generic Your call has been forward to an automated voice message system recording. If you use your phone for work, create a personalized greeting that tells people you're a professional. If your phone is for personal use, create a personalized greeting so people hear your voice and know they called the right number.
Setting up your voicemail. 1. Press and hold 1. 2. Enter your password if prompted. If you are asked for a password the first time you access your voicemail, just enter the last four digits of your phone number. 3. Follow the directions to set up your password. 4.

How to Change the Outgoing Voicemail Message on IPhone (iOS 9 or later) Tap the phone icon (the same one you use to make a call). Tap the voicemail icon in the lower right of the screen (it’s the one that looks like an old-fashioned cassette tape).
When you first set up your Android phone, it may ask you to record your greeting for the caller. You can also keep the default greeting stored on the device. But, in future, if you want to customize the Voicemail greeting or want to create a new one, then you may need to delete the old recordings from Android. It may not be necessary depending on the Android device model & version; it’s good to know the process.