Why: Warning Voicemail Full – iPhone
Are you receiving a warning message in your iPhone voice mailbox that you reached a percent full of space?
Voicemail 95% Full
If you are deleting your voicemail messages in Deleted Messages on your iPhone, it may take some time to update the deleted messages on your iPhone carrier server. In this case, the warning message will continue to appear until after the voicemail messages are deleted on your carrier (Verizon, AT&T, Sprint, etc.) server.
All else, if you continue receiving the voicemail warning message, perform a Network Reset on your iPhone. On your iPhone, go to Settings -> General -> Reset -> Reset Network Settings.