As with any utility you may purchase, or for that matter things like your domain name, web hosting, etc. it really just gets down to how well you trust the provider. These are not the areas where you want to cut corners, but it's not necessarily hard to find providers who have a good reputation and which won't fold overnight. Two suggestions I would have for small business VoIP would be:
- Google Voice
- Grasshopper.com
Either of these are going to work well with the primarily cell-phone driven situation and give you an app which will let you transparently make and receive calls and texts without exposing your personal cell number. And these are just the first two examples off the top of my head.