It’s a good practice for each of your team members to have their own personal business phone numbers. There are many reasons why they shouldn’t use their personal cell phone number for business, so you’ll want to give them their own phone number through your VoIP provider or phone system.
We managed to discover some of the most common causes that will trigger this particular issue. We have done this by looking at various user reports and the repair strategies that they used to get the issue resolved. Here is a shortlist with common scenarios that have the chance of triggering this particular error message:

It drives me mad when I get put through to someone’s voicemail system – only to hear a message telling me that the voicemail-box is full, so I should just bugger off and leave them alone. The number of messages that your voicemail system can store will vary from network to network – Virgin provides a free voicemail option that stores just 10 messages at a time, but you have to pay extra for its VoiceMail Plus service, which stores up to 30 messages.
If a phone rings once and then goes to voicemail or rings only briefly, it You can't definitively know the cause of a phone barely ringing and going to voicemail because neither a phone company nor the phone's operating system provider will tell you if your phone number is blocked.