I had been invited to a conference call yesterday but wasn't looking forward to it because I didn't want to burn-up a bunch of minutes. Yes. I'm still on a plan with a defined number of minutes. While I have unlimited data (in theory) the minutes are finite and they're shared among the whole family. I'm far and away the biggest user of minutes and while my Fave Five solves the problem of calling the top five people outside the T-Mobile network, it does nothing for me in these kinds of circumstances. I found myself wishing that Skype supported toll-free calling to toll-free numbers like my SIP provider does.
Hang on a second. I had never tried it nor even googled it. Sure enough, I found a blog post on the Skype blog from five years ago(!) where they announced that Skype users could call toll free numbers in various countries at no additional charge. Bingo!!! I was in like Flint.
"But how," you might ask, "can Skype offer toll-free calling to toll-free numbers? Easy — the power of SIP. Before explaining how Skype can let you make toll-free calls without charge, I'll explain how to make the call.
Making a toll-free call via Skype is very easy. On your computer using the Skype software, just click the dial pad and start typing the number (or clicking the dial-pad numbers) and hit send. The call will connect. If Skype recognizes the number as a toll-free number the rate will show as $0.00 (€0.00 or other currency). Skype recognize many toll-free prefixes throughout the world.
On the N900 you have to make one small change: prefix the toll-free number with the plus (+) sign: e.g., +18005551212. Since Skype can call both users and phone numbers the client on the N900 needs a clue where to route the call. In the display on the N900 the phone application will show the rate the same way. As long as it comes up as $0.00 you're call is charge free. Now for "how" Skype do it.
I mentioned my SIP provider earlier. I have several SIP accounts with various providers where I pay for minutes to call foreign lands. I have multiple accounts because rates to call these foreign lands often vary — significantly. When my sister was living in Guatemala I found one provider who could offer me single-digit rates compared to the obscene 30+ cents per minute most providers were offering. Another provider has excellent rates to Mexico and another offers free calling to many countries for a set period of time. Of course signing-up with multiple providers is easy but managing them on your phone is another matter. Unless you sign-up with a company that will look at all your SIP plans and route the call to the provider with the lowest rate. Voxalot offer just such a service.
While organizing my accounts I discovered they would route most toll-free calls along a pure SIP route so the calls would go through without charge. A bit of research and applied logic revealed why this was possible: the phone companies can increase their margins insanely if they can route toll-free calls (indeed all calls) along SIP routes. Toll-free calls aren't really toll-free. They're toll-free to the caller. Remember way back in the day when we paid to call the next state, county, city and sometimes next exchange within a city? Toll-free numbers allowed businesses to offer a consolidated, cost-free number their customers could call and spend money. Now that we all have cell phones and even land lines with unlimited national calling the "800" number is a bit of an anachronism. Not entirely but quite a bit.
Toll-free numbers still exist and the person or company with the toll-free number still pay the bill but now the phone companies have reduced their costs by routing the calls across the internet (or more likely proprietary networks that also connect to the internet). Other than the basic costs of connecting their phone-routing centers to the internet, toll-free calls are basically cost-free to the phone companies which means their profit margins have gone up. Way up.
To keep their costs low the phone companies allow practically any SIP client to connect to those same toll-free pathways. Why? It saves them money. They still bill the customer — whether at full-tariff or reduced-tarrif rates — for the call. Why not let SIP clients connect and keep the call off their public-switched network? It's a win-win-win for the SIP user, "800" customer and the phone company.
Tying all this info together it's obvious how Skype can give you toll-free calling at no cost: either they're connected directly to the phone companies or, more likely, they're routing it via SIP the same way Voxalot and other companies offer toll-free calling at no charge. Skype can offer it to their customers on the same infrastructure they've already invested in and therefore at the same cost they offer free calling to all their Skype customers when calling other Skype customers.
Why they do that I'm sure still baffles E-Bay's Board of Directors
 I'm sure the N900 software or Skype could quickly determine it's a toll-free call rather than a user name. My guess is the "+" sign just short-circuits the process and makes it faster.
 Undoubtedly the SIP call and a call initiated through the public-switched network travel along the same paths but those connections to the POTS lines are finite. SIP calls free-up those connections.