After you insert a SIM, the N900 sets up GPRS using the MCC and MNC values from your network to look up preset values in the file /etc/operator_settings. At least, that's what it looks like.

Unfortunately the values in that file are wrong for some SIMS.

For me, it produced the wrong values for both Vodafone UK PAYG (it chose access point "Internet" which doesn't work), and for GiffGaff (which it wrongly gave O2 settings that don't work on GiffGaff.)

A big problem is that MCC and MNC values are not sufficient to identify which GPRS settings will work, so the concept of this file on the N900 is flawed.

For Vodafone UK PAYG, use these settings:

Access point: pp.vodafone.co.uk
Username: web
Password: web

There is no need for a proxy, although in theory a good proxy choice might make browsing better, or worse.

For Vodafone UK Contract, some people have reported success using access point "pp.internet" and the same username and password as above. There may be some confusion between phone contracts and data cards (contract or PAYG), though.

Isn't it possible to read the right setting from the SIM itself?

Does anyone know of a tool for displaying SIM contents?