Network 3 sim in unlocked phone

[clayt74]

Can also depend on the age of the sim! 3 where the first to bring out what they called a U sim(not to be confused with the new micro sim)
The U sim was the same size as the old sims but would only work in a 3 phone regardless of if the phone was unlocked or not.
Try a different sim and if it works 3 should replace your existing sim and no for free