Nokia N78 GPRS problem

Troubleshoot from the basics. Try putting the original SIM card back in and see if you get the same error message, different error message or if it works normally. I would think that if you change the SIM card you are going to have to reconfigure it to with with the new one unless there is conflicting information stored on the card. Calling customer service isn't always the best way to go about things, some of the call techs have no idea what they are talking about.

Nokia phones seem to have a lot of issues when swapping SIM cards. But some of the settings are from within the phone so there could be a conflict there. Try using the first SIM and seeing if that allows you to access/ use the GPS.

If not, try asking the people on the message boards here:
http://europe.nokia.com/index.html

They have a lot of very helpful people that can give you more detailed information if you can't get the SIM card issue addressed.

Here's the thing: when you set up your GPRS, you only set it up for the simcard that you are using at that time. If you change SIMs, you'll have to set it up again. (This is 50% true if the 2 SIMs are from the same provider and 110% true if they're from different ones).

Also, it would do you good to check on your list of internet profiles which one you are using. (Usually under settings when you go into the phone's browser.)