Carla,
I have researched this much tonight, but I did look extensively at what is related when I opened my practice. Even if what I say is off base, hopefully I can help you by at least saying, "Be careful."
First, I believe most business phones are digital. Try taking your home phone and plugging it into your office jack. I doubt it will work and vice versa. Your phone system in your office runs back to a rather expensive converter for lack of a better term.
What you are referring to runs over the Internet. It is VoIP, not just a digital phone system. The digital phone system in your office is something like Verizon or AT&T or whatever. As my phone vendor said to me, "When was the last time you picked up your receiver and didn't hear a dial tone?" Correct. Almost never.
From what I read on Costco.com, it sounds more like what you would have at home like Vonage. It would just simply be using a device to change your voice (analog) to packets (digital).
Voice over Internet Protocol or IP which is just over your broadband connection depends on several things. It depends mainly on bandwidth. You must have a lot of bandwidth down, but you ALSO must have a ton of bandwidth UP. Your patients will be very unhappy with anything less than around 754 up and down. Since, you are sending packets, they will drop, get lost, be sniffed, etc. If something happens, who will be there for support?
With business VoIP, the cost as always is much higher. Each "seat" which is a phone costs, say, $40 per month depending on the features. So, you add an employee and need another phone, it's $40 more per month. So, two phones = 80 but 10 phones = $400. But, you only need one cable line. A very good VoIP would use a T1 to the main trunk of the Internet. But, to give an example, even though I never purchased the main piece of equipment for VoIP (I forget the term), I did know I needed a VoIP ready switch. Most switches like Linksys or NetGear for an office LAN can be bought for $60 to $150. My swithc was over $1,000. Works great, but I didn't need it. I ended up not going with VoIP.
If you go with business VoIP, which is highly recommended, you can either go with your own VoIP setup in your office, cheaper in the long run, but you have to maintain it or pay someone to do it for you or you go with Hosted VoIP. This is where a third party company hosts it for you and has all of the extremely expensive equipment such as switching, etc. Think of a small phone company -- actually smaller.
Now, with real VoIP, not Vonage, you can do amazing things. Everyone will eventually go VoIP, I want to. I can't even explain the amazing things you can do with VoIP and your computer. Just to give an example, say you have four phones in your office. These are VoIP phones, mind you. And, your phone number is 207-555-1111. You can pick up a phone, take it to Branson, MO or Japan, plug it into any Internet jack and people can call you at 555-1111. It's like an office call. Your secretary could talk to you like you were two offices over.
I don't want to burst your bubble. And, again, maybe this is something worth looking into. But, I will give you a website to look over which has lots of information on this stuff. Maybe it is what you are looking for. But, at least you can compare it to the Costco phone system.
With consumer VoIP, you have the converter from analog to digital much like a modem. With commercial, it is Ethernet based. Keep in mind again that your call quality is based heavily on your bandwidth. With commercial VoIP, your system has a component which protects your phones, so if you set it at 700 and your employee starts to download those MP3s, it will not let all of your bandwidth go to the download.
Anyway, check out:
http://www.voipreview.org/Business_Telephone_Systems/BusinessArticles.aspx
