http://cnac.ca/data/ac873.htm indicates "NPA 873 will serve the same geographic region as the existing 819 in Québec (See Planning Letter PL-NANP-405). Central Office (NXX) Codes may be assigned after December 01, 2012, however, assigned codes cannot be activated in the PSTN prior to June 01, 2013."

There are therefore no legit +1-873- numbers at the current time; all that appears in the official listing of telephone exchanges are the standard reservations of test numbers (958-, 959-), information (x11, 555-), local, adjacent or proposed area codes (819-, 613-...) and assorted special numbers such as 410- (seven-digit regional tollfree) or 976- (pay-per-call ripoff numbers) in an otherwise vacant area code.

If you receive a call from area code +1-873 prior to 2013? It's bogus. Don't even pick up the 'phone.

Just a please call this number message. No reason, no identification of the caller. And I have no idea what's going on except that they have called at least a dozen times in the last few weeks.