yes calls and txts work fine, i didn't even need to change the setting for them to work :-\
The settings are only for MMS and internet - calling and texting always works automatically!
For a giffgaff sim with £5 FREE CREDIT when you activate, click here
Go to settings->Wireless and Networks->Mobile networks->Accesspoint names
Use the settings button on your phone and a menu will come up. Click "Reset to default"
This resets your APNs. Then go to the market (with wifi connection) and download "giffgaffapn" app and OPEN it and go through it and install both internet and MMS settings from it.
Your good to go (I hope)