some said Telco set need to return to Telco after contract period. else need to pay for penalty.

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.

During contract, depend on the phone, they have the authority to control the phone. I remember green telco sets got some stuff cannot access but forget which one but it is in Android though
After contract ends, you own the phone
If they perform locking, you can actually blast them for unauthorized lock