Area code 787

Telephone numbers in Puerto Rico are assigned under the North American Numbering Plan (NANP).

Prior to March 1, 1996, Puerto Rico was one of many Caribbean islands served by area code 809. On that date Puerto Rico was assigned the new area code 787 (with the mnemonic PUR or PTR).

As elsewhere in the NANP region, increasing use of modems and mobile telephones strained the supply of seven-digit numbers within area 787, and an overlay plan came into effect in or about September 2001. Puerto Rico now has two area codes, 787 and 939. The new code is used mostly for wireless phones.

Because of the overlay, the area code must be dialed for all calls.

...
Wikipedia