Most countries require the use of a so called Trunk Code for national calls. A Trunk Code is a single digit (or, in some cases such as Hungary or Mexico, 2-3 digits), often 0 (zero) added before the actual phone number in its national dialing format. This trunk code MUST BE LEFT OUT when dialing in from abroad.
Most telephone service providers (fixed and wireless) provide the option to block international calling. Some even have this as a default feature. So, before trying to place a direct international call make sure you are permitted to do it.
In most countries outside the US cell phone numbers have a national dialing format, usually being allocated a starting digit not used for landlines. No area codes are added to a cell number. For most countries we provide the dialing format for mobile numbers. Again, do not add area codes to cell numbers unless told so (there are a few countries such as Mexico that do use area codes for cell numbers - this is clearly pointed out on the pages in question).
A feature offered by some VoIP service providers is to call internationally WITHOUT DIALING 011 FIRST. Having this feature turned on usually means that you must leave out the 011 Exit Code or your will not be able to call.