Rob, do you know whether, with OnSIP, I can assign one extension to multiple phones? That would really help with the monthly costs if I could have all the little-used phones in the non-office locations share an extension.
Yes. Multiple devices can share the same extension, no problem.
My extension, for example, rings on my desk at work, my desk at home, and my
Android SIP client all at once.
As for voip.ms, I believe you can make a PBX-like experience using their service and sub-accounts. We did used to have a single SIP number hosted there, so I am a believer in their service. But, it's more geared toward people configuring a single line, or plugging the SIP credentials into their own PBX (Asterisk, etc) as a trunk. As a PBX, OnSIP will be a more complete solution, but at a relatively higher price. Their interface is also more user-friendly and your client might actually be able to take care of simple things themselves. However, voip.ms could be a lot cheaper.