Only unsafe when you didn't pay your bill for more than x number of months as Telco can perform Phone Locking as their last resort.

You can check with the counter on this.

Other than that you are the one who owns the phone but you need to be in full tenure for the contract period as per signed if not there will be additional penalty charges.

And also if you are planning to resell the phone after that, you are pretty much in dead end as most buyer will avoid telco set especially those that is under 2 years unless you can share proof that you had purchased fully or proof that the phone is already out of contract.

There is no rights for any third party to access your phone data especially when we talk about iPhone which the data is strictly locked to the owner.

The only way for third party to access your personal data is either the owner shared the password to another person such as Family Member or by sharing your icloud detail to third party.