I have a 3 year old phone on monthly contract,I want to save some money,can I just cancel?what happens to the phone?can I still use it?if so how?,I renewed over a year ago but it seems that I am paying over £20 per month.

Phone contracts are normally 12 months so check, if you signed for 18 or 24 months you will be in breach of contract, they will expect the phone back and payment for the remaining months.

If you have say 3 -9 months left it is normally cheaper to see out the contract then keep the phone.

To continue using the phone I think you will need a new sim card, or ask for a number from the service provider (could be IMEI) ?

The Dr on here is normally pretty good at this stuff I think.

Did you get the phone new 3 years ago and just re-new the contract?
If so, just phone up the company and ask them what you can do. They may be able to cheapen the deal, depending on your usage, or even swap you to a pay-as-you-go customer.
You are always best to ask the company, otherwise you can get into credit problems.