Website: https://support.digium.com/community/s/article/How-do-I-record-my-Voicemail-Greeting-over-the-Phone
Dial your mobile, wait for it to go to answer phone and hit the star key, a prompt will ask you to enter your pin code that you set up previously, followed by the hash (#) key. So if you have no credit to access voicemail to set up a pin your kinda stumped.

By using the reset network settings, an effectiveoption to solve network related issues, you can fix allthese problems by simply resetting the networksettings of your iPhone as it will clear all the networksettings, current cellular network settings, saved Wi-Finetwork settings, Wi-Fi passwords, and VPNsettings How do you reset your voicemail greeting? From the Home screen, tap the Phone icon . Tap Voicemail then tap Greeting (upper-left). Greeting islocated in the top-left corner of screen. Tap Custom to record a greeting. Enabled when a checkmark ispresent. Tap Record to begin recording the custom greeting message. Tap Stop to end recording then tap Save. How do you change voicemail settings on iPhone? Go to the Phone app, then tap the Voicemail tab. Tap Set Up Now. Create a voicemail password, then tap Done. Enter your password again to confirm it, then tap Done. Select Custom or Default. If you choose Custom, you can recorda new greeting. Tap Done to save your greeting.
Enter a name for the new greeting and then tap Save. Change your greeting. To use a different greeting that you've already recorded or switch back to the default greeting: On your Android device, open the Google Voice app . At the top left, tap Menu Settings. In the Voicemail section, tap Voicemail greeting.