The Instinct is a CDMA phone - most of Europe uses the GSM standard for mobile phones. It will not work.
Sprint currently has two "World Phones" that will work in Europe. The Samsung Ace (Windows Mobile phone) and the Blackberry 8830. The Nextel Direct Connect i920, is also an international phone, however, I'm not sure if it works in Europe or not.
We also provide rental phones that are GSM only for use when you travel. If you are planning on going out of the country and would like to find out more information on them, you can call the Worldwide Care team at 1-888-226-7212, or you can visit this page on the Sprint website: Traveling Internationally.
It appears most CDMA phones should work in Mexico - this link shows the coverage types and recommended phones.
Side note - I had no idea this site was still live; I wrote it about 6 years ago. I would have sworn it was turned down years ago when we merged with Nextel.
Another side note - if that link doesn't work, go here:
and choose North America, then Mexico.