Early termination of a contract with iD results in a termination fee. To check if there is an early termination fee, text INFO to 85075 from the phone with the iD SIM. The reply will provide this information if 30-days notice of cancellation is given. If you give 30 days notice to terminate the contract and number will be ceased.
As you are paying your contract off you get to keep the phone.
If you want to keep your number you can request a PAC code and transfer the number to another network - when the number transfers this will be classed as your termination date.