I can't really answer about prepaid cell phones, but I have another idea you might find less *risky*...
First, if you have a landline still upstairs in the office, then why not just buy another handset for downstairs? You know, where you have the base station and multiple handsets that you just plug the power cord into a wall socket to recharge the individual handsets?
If you don't want to do that b/c you don't want them answering your business line, you could probably set it up to have a different ring tone.
Alternatively, you could get an Ooma and do the same thing. The monthly fee is just taxes once you buy the Ooma itself. Ooma is VoIP service. You can set up all kinds of ring tones. Basic service averages $4/month, and you use it just like a landline. Multiple handsets, etc.
ETA after your SWH:
So, since you have NO landline, maybe look at switching from Vonage to Ooma. From my research, it gets better reviews. The sound quality is top notch, and while yes, you'd have to buy the Ooma box thingee initially, it's a one time cost, and it is like a router, in that you can plug any phone you want to into it. We just use the set we already had (a base station with 3 additional handsets). We still have the additional handsets plugged all over the house and there is no change in how they function--except it comes through VoIP instead of a hardwire landline that costs $70/month.
It might cost a little up front, but in the long run, it pays for itself and has great features. AND, you can add an additional phone number, not just the ported one. So you could actually have an office number and a home number if you wanted.
Just a thought.
Yes, there's a start up cost (but there is with a prepaid cell phone as well, and you have the issue of keeping up with it and passing it around to your sitters or whatever, etc.). With Ooma, they wouldn't even know you didn't have a landline. At least go to their website and check it out. We love it.
http://getvoip.com/blog/2014/02/18/ooma-vs-vonage/
http://www.ooma.com/
And don't be confused by all the *extras* you can purchase with the teleo for Ooma. If you have the right stuff at home already, you only need the Ooma teleo itself, not all the wifi jacks and other accessories. All we bought was the teleo (and it plugs into the line between the DSL modem and the router). Then we plugged our existing phone base/answering system into the Teleo. That was it. All the other hand sets we had plugged in (not into phone jacks, but into power outlets only) didn't need to have anything done to them at all.
I can blacklist calls (and the system even has it's own database of telemarketers and junk callers that it filters out if you want), forward calls to my cell phone, program it to turn the ringer off for certain time frames, or have different ring tones for different callers or for the 2nd phone number. It's pretty sweet. I don't use half the features... ok, not even half... but they are there if I need them.
I don't know if international calls are an issue for you since you use your current line for business. But if not, and you are paying a fair amount for monthly service, then you'll make back the cost of the teleo unit in no time. If you go with basic service, the charges are truly less than $4 per month. With premier service (all the bells and whistles) it comes out to about $14/month (TOTAL). The Teleo was about $140, and you can order online, or go into a store an pick one up in person (I think Best Buy had them when we got ours).