This is not the means by which formal complaints can be raised, however by means of general advice.
EE's traditional contracts have no concept of "paying off" a phone or separating elements of your service. You have one single monthly payment covering ll your services, which you contractually agreed to pay on a rolling ongoing basis, but for at least the duration of your minimum-term. You're free to change your plan to a SIM-only once the minimum-term has ended, but this has to be your choice and cannot be imposed.
For several years now, EE have sent text messages when a phone is approaching the end of minimum-term, outlining the options available. These would include upgrading to a newer phone, changing to a SIM-only plan to reduce costs, or remaining as-is. The ultimate choice simply must be that of the account holder, and if no change is requested, none will be made.
EE's newer FlexPay plans have two distinct & separate contracts, each with their own payments. You have a fixed-term finance loan for your phone, which naturally ends once repaid. You then have a separate minimum-term-based plan for your airtime. You may wish to consider this type of plan in the future.
Formal complaints can be raised via https://ee.co.uk/help/contact-ee/complaint but I would strongly recommend bearing the above comments in mind before doing so. If you're on a traditional plan and are merely unhappy at having gone beyond your minimum-term without changing your plan, no repayment is due to you.
