As per the contract your discount expires at month 18 however there is a reason for this.
Due to tariffs changing all the time, handsets needing upgrading due to old age etc, this is when your account is given a complete review to see if there are any changes that need doing.
This is when you will receive a call from the retention's department to give your account a full review and discuss all the options available to you regarding new tariffs, potential new handsets, SIMO deals etc. Typically they try and call you prior to the 18 month mark, so you have the chance to ask as many questions as you like and inform them about any changes you would like to make ie, switch networks if this is possible, potentially drop a line that you arent using etc.
They are there to assist you as much as possible so dont be afraid to ask and they will advise if your proposed changes are viable.