Sim cards and carriers, discussion

[LinAdmin2]

You also need to know which frequencies the cells in your area are using - some carriers use a mixture of supported and unsupported frequencies, and some cells don't use any of the supported frequencies.

IMHO it is not only depending on the frequency bands, but more on the details of the handshakes in the triangle built by Simcard, moden and the software feeding the antenna of the provider.
The same OS image allowing perfect telephone and sms connections in Austria does not work in neighbor Switzerland...