Prepaid. Every time I and my wife travel abroad callers can't leave a voicemail. This happens to both lines.
If someone dials our numbers they hear: "We are sorry, the number you have dialed have calling restrictions that have prevented the completion of your call. Announcement 19".
At the minute we land in the US and everything is ok.
It is absolutely normal for a voicemail to be functioning while the phone is out of service area or on airplane mode.
I tried contacted the support numerous times in the past 3 years and was on the phone/ chat with so many agents. They suggested replacing the sim back in the days, all types of reset at both our end and their end, change the number temporary. They always promise the issue will be escalated and nothing. Any suggestions besides changing the carrier?