What country is this number from 011 52 55 5555 xxxx?

011 - international access code from US/Canada/NANPA

+52 - country code for Mexico

55 - area code for Mexico City

5555 xxxx - number within Mexico City

If in fact 5555-xxxx is a valid number in Mexico City, it would be dialed as 011-52-55-5555-xxxx if it is a landline, or as 011-52-1-55-5555-xxxx if it is a mobile phone (cell phone).

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)