I tried calling Cathay Pacific's eService Center Hotline at +800 2747-2200 from my T-Mobile cell phone today (while I was in the U.S.). If that number looks strange to you, it's what's called a Universal International Freephone Number (UIFN) (the ITU has more official information about UIFNs). Basically, a UIFN works like a 1-800 phone number in the United States except that it is international. The "800" part of the number is, for most intents, identical to the country code in a normal international phone number. That means that you can call a +800 number from anywhere in the world and it will be a free phone call (for you, the recipient of the call still pays of course).
That's the theory at least. In practice, when I call that number from my cell phone I get a message stating "This service is restricted or unavailable. Please contact customer care." So I called up T-Mobile. I spent an hour on the phone being transferred to various support representatives who didn't know what I was talking nor what they were talking about. Not only did I have to explain to each rep what a +800 number was, and that it was not a 1-800 number that I happened to write down incorrectly, but also various basics of international phone number dialing, e.g. that "011" and "+" are equivalent dialing prefixes in the U.S., that I really shouldn't have to explain to a support person from a phone company. What's worse, the database of country codes that the T-Mobile support reps use internally doesn't even list 800 as a known country code.
In the end, I finally got someone who explained to me that that message is the same one that you receive if you try to dial a 1-900 number from your cell phone (T-Mobile blocks 1-900 numbers from cell phones). Thus, I was told, T-Mobile blocks +800 numbers and there's no way around it. This at least was a sensible explanation of the problem I was seeing. I tried to probe deeper about why T-Mobile would feel the need to block +800 numbers, but all I got was some uninformed rambling about T-Mobile wanting me to pay for my phone calls rather than go around them and get calls for free. When I asked how this was different from 1-800 numbers, which are not blocked by T-Mobile, the support person fell into an incomprehensible babble. At that point I gave up playing the "transfer me to your supervisor" game and resigned myself to the fact that I'd never be able to dial a +800 number from my cell phone.