You do not get to test timefor replacement, when you take out a contract you normally get 7 days to test phone and network and if you are not happy you can return and cancel contract.

You have 28 days from the 1st day you get the phone for replacement or repair, after 28 days its just repair untill your contract runs out.
If you have a nokia phone its 2 years so if you get a contract for 18 months you still have another 6 months through nokia to get it fixed