We are a small business with a 2D fiber connection -200GB/month, 100up/100down, copper phone line.  Our phone system is simple - all extensions are dumb phones all in parallel.  We have just moved out of a 24-month contract and are costs are now $134+/mo.  Calls are on top of this but we are not big users.  2D have sharpened their pencils a bit and are trying to get us to sign up for another 24-month contract.  They are offering the same except that the 200G/month becomes unlimited, and the cost is 125+.  They are also offering a VoIP option for $98+ which also offers unlimited calling. Like I say, we are not big voice users, but a that probably represents a further $10-20 per month.

We don't have an issue with VoIP - I have it at home and it hasn't missed a beat, though I have heard of people that have had issues. The Fritzbox at home has no difficulty with a half dozen or so analogue phones plus a fax connected to its POTS port.  

At work however we have a monitored alarm.  The alarm was put in when the building was constructed 25-odd years ago.

I have heard of boxes that enable monitored alarms to play nicely with VoIP, but I'm keen to hear of anyone's experiences here.