What country code is 255?

In its entirety, that is not a valid country code.

What you may be looking for is the answer to puzzle 10 from God Tower (see related link.) You have to search each of the country codes separately so it works out like this:

255 = Tanzania

968 = Oman

977= Nepal

66 = Thailand

247 = Ascension

262 = Reunion Island

599 = Netherlands Antilles

The first letter of each country code gives you the answer "tontarn" which is the answer to the puzzle.