Bill: Just one note to consider; if you phone does not support the GSM frequencies in use across Europe, it will not work. Check your U.S. GSM phone to make certain it is either a Quad-Band or Tri-Band phone. My US t-Mobile handset is a Quad-Band phone. But you cannot take it for granted - must verify.
Here is a portion of another post I made on the subject:
"This last bit is the key issue for basic voice / text handsets. Basically voice/text only phones are sold as dual band, tri-band, or quad band models. Across the world, except for Japan, there are four (4) GSM frequencies used. In North America, GSM phones used the 850 Mhz and 1900 Mhz frequency bands. In the REST of THE WORLD (except Japan), GSM phones use the 900 Mhz and 1800 Mhz frequency bands. Japan uses a fifth, non-standard frequency."
The full post can be found at:
http://www.caminodesantiago.me/community/threads/getting-a-cell-phone-in-spain.16546/
Thus, a
quad-band phone can access service at: 850, 900, 1800, and 1900 Mhz. A
tri-band phone usually accesses both North American frequencies and ONE of the European frequencies 900 or 1800 Mhz.
Once you sort this issue out, then I recommend Vodaphone coverage for Spain. Orange is very good too. t-Mobile seems to work everywhere.
I hope this helps.