It sounds like the device does not qualify for a SIM unlock from T-Mobile.  If you are not the original owner and you don't know them.  More than likely, they would need to make the SIM Unlock request because their account and the device usage for that account would need to meet all of the requirements for the SIM Unlock Policy.

https://www.t-mobile.com/responsibility/consumer-info/policies/sim-unlock-policy

Thank you for your quick answer

so, it is about policy… a phone from 2016 should have been unlocked automatically…

would it help to send a message to Sprint?

Sprint no longer exists and hasn't for years now.  Devices automatically SIM unlocking is a fairly new policy that came in late last year.  Because it isn't the original owner with the device purchased on the account requesting it.  It does not meet the SIM Unlock Policy requirements and the usage requirements can't be verified either.

Hello I have been trying to get in contact with Sprint to unlock my phone and Tmobile sends me to a number that does not work. I get a message that this Sprint number is no longer in service. Someone please help. 800 937 8997

Are you currently a T-Mobile USA customer?  Does your account and device meet the SIM Unlock Policy requirements?

https://www.t-mobile.com/responsibility/consumer-info/policies/sim-unlock-policy

Hello I am compliant with t-mobile policy in unlocking a phone. I have tried calling sprint and the number says it is no longer in service someone help. 

Sprint was dissolved in a merger with T-Mobile years ago.  Sprint does not exist anymore and hasn't for some time now.

Sprint was not dissolved they are active. T-mobile twice has sent me to a number that does not work. A live representative speaking to me telling me that Sprint has to unlock my phone. 

Here's Sprint’s support page.

https://www.sprint.com/en/support/contact-us.html

Um it doesn't work. Believe me we are talking hours of going in circles. This is their support number it doesn't work. 8882114727

Are you calling from within the USA or outside of the USA?

M'y iphone 8plusis carrier Lock IMEi is 356117095347532 carrier Sprint 

Are you a Tmobile / Sprint customer?

Apple iPhone 13 Pro Bleu alpin 128 Go

IMEI 352806844458822. a year after purchasing it from an online store, my phone stopped accepting SIM cards Now I don’t know what to do, help me!!! us sprint unlock !!!

Contact the online store that sold it to you. The phone was probably jailbroked and relocked after a OS update.

I tried to find the seller without success! What are the options ? and how could they hack my phone?

Someone jailbroke the phone to mimic an unlocked phone but once the OS is upgraded the phone will relock.. The person who sold you the phone most likely did it.

You can check the status of the imei number here.

https://www.t-mobile.com/resources/bring-your-own-phone

• This device is fully compatible with T-Mobile’s network technology. Tell me more
• We can’t confirm that this device is unblocked. Please contact the original wireless carrier for information about the device’s status Or call us now to complete your order 855-806-2656.
• Your device is eSIM-compatible and can be activated right away. What is eSIM compatibility?

Esim is when you don't need a physical Sim.

Hello , buy an iphone 13 pro max online 128 gb graphite color it says the sim is blocked in case the company the imei is 350019048765289 please give me a solution to unlock the cell phone