I knew that 555-numbers exist in the 800 area code (Canada Direct is 800-555-1111 from the USA, for instance) but I’m not sure about the other tollfree area codes (888, 877, 866, and soon 855). But I thought that they were only used in tollfree area codes.
However, after digging through the North American Numbering Plan Administration’s numbering-resource pages (look for “555-NXX Codes” on the NANPA site), I followed links to the Alliance for Telecommunications Industry Solutions, and found the following Microsoft Word document, 555 NXX Assignment Guidelines.
It “specifies guidelines for the assignment of line numbers within the 555 NXX [exchange] code, hereafter referred to as ‘555 numbers’ within [the] guidelines. The intended use for 555 numbers for which these guidelines apply include the provisioning of information services but may include a broad range of existing and future services as well.” and “These guidelines apply only to the assignment of 555 numbers in geographic NPAs [area codes].”
So it looks like we can now have “<area code>+555+<four digits>” as a valid phone number. But it would belong to some “information provider”; 555-numbers presumably would not be issued to residences.
There’s a lot more in the documant about getting “national” versus “local” 555-number assignments: ways to reserve the same four digits after 555 in a number of different area codes. How many companies actually use these numbers, and how they bill for their “information services”, is another question.
However, going down through the document, I find Section 4.6:
“A block of one hundred (100) 555 line numbers will be reserved as fictitious non-working numbers for use by the entertainment and advertising industries. These specific numbers are 555-01XX, i.e., numbers between and including 555-0100 and 555-0199.” (italics mine)
So there you are. Just make sure that your fictitious phone number is between 555-0100 and 555-0199 inclusive, in any area code, and it will never work. 
On the other hand, area code 555 is ‘unassignable due to conflict’, at least according to Area-code Info code lists. So maybe you could use it.