radioguy wrote:
I have a GSM cell phone.

Despite what people in other contries say about just switching sin
cards when you go to another country,

that does not work.

Actually it works fine. The problem that you are having is because you
are trying to put a foreign SIM in a phone that was sold to you at a discount
by a cellular service provider. In order to prevent you from taking their
phone and selling it on eBay, they lock it to SIMs provided by them.

If you were to buy an unlocked phone, or have your service provider (SP)
remove the SP lock, then it would would fine.

People who travel know this and either buy an unlocked US GSM phone
(850/1900mHz), or buy a multiband phone that includes the US bands.
If you have a EU only (900/1800) phone, you can often rent a US one
for little money or free from your SP.

There also is a question about 3g Networks, there are two different bands
used in different places. I think the US uses 1900mHz, the EU etc use 2100 mHz.

As for coverage, there are in the US two GSM bands. 1900mHz bands cover
about 80% of the population, but around 20% of the area. 850mHz covers
the old AT&T Wirless network coverage area, which is more than 99% of
the US.

However there is no such thing as 100% coverage anywhere with cell phones,
if you want that you have to use a satellite phone. The best are combined
satellite and quad band GSM with roaming agreements, so when you are within
the coverage area of any GSM network you use that, but if you are in one
of the few (in the US), or many outside, areas without GSM coverage, you
use the satellite.

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM