It's really up to them to unlock it since they most likely subsidised the price that you paid for it.
Usually a carrier will do this if you have had it for a certain period of time, are out of contract or bought a specific amount of pay as you go credit.
Will they not unlock it at all or only if you pay something?
If all else fails it may be possible to get it unlocked unofficially online or in a phone store.
What make/model of phone is it?
Hope this helps.
