Unlock phones do not always include 3G
A lot of people now use the 3G network including Video Calling so many of these people buy old phones thinking to themselves they can use their usual 3G Sim card in them when unlocked for a small fee.
This is not so. Old phones were not catered for 3G because at the time of the production, there was no 3G!
If you are a 3G user yourself, be sure you get a compatible phone and not just any old one lying around in the garage. This is more important when you plan to use the second phone for emergencies only.