All that makes configuring and managing home routers very difficult for IT personnel, but they'll still be faced with that task because if employees run into conversation problems over their softphones, their first call will be to the IT help desk. Shunting them off to the VoIP provider, or worse, the ISP isn't a good idea either. It'll likely cause employee frustration since those outfits won't be familiar with your company and they'll either refuse support or it'll take a very long time.
Consider adding 'You can also email your query to us at [insert email address here]. These queries will be answered within [insert time frame].' Again, if you intend to make a promise to your customer in a voicemail, ensure you keep it.

For many, checking voicemails is a productivity killer. It's a slow and cumbersome process that makes the user feel like they are using ancient tech to do their job when there are more efficient messaging services available.
Voicemail essentials: Identify yourself because you need to reassure the client that they have called the correct number. Clients will be a lot less likely to leave a message if they wonder who will get it. Use a warm friendly voice to say something like, “You have reached xxxx, licensed mental health counselor.”