iPhone 15 Network Lock

If you are a T-Mobile USA customer and your account and device meet all of the SIM Unlock Policy requirements.  You can reach out to Customer Care by dialing 611 from your T-Mobile number and verify your device meets all of the SIM Unlock Policy requirements, then request the SIM unlock. 

SIM Unlock Policy | Unlock Your Mobile Wireless Device | T-Mobile https://www.t-mobile.com/responsibility/consumer-info/policies/sim-unlock-policy 

If you don't meet all of the SIM Unlock Policy requiremebts for your account and devuce.  There is nothing T-Mobile can do.  iPhone SIM unlocks take about 3 weeks to process because the request is sent to Apple by T-Mobile for Apple to fulfill. 

Are you a T-Mobile customer ? if so there's requirements that need to be met.