You need to try using messenger on one of tmobile's social media platforms to see if they can help you with this.

This is basically a user to user forum with some tmobile moderation that don't have account access.

Is it so complicated that they verify that it is free of payments and with the IMEI send the unlock code?

we have all the information of the deceased client

Unfortunately that's the only way to resolve the issue.

The family member died and her partner went to a tmobile store to resolve and justify this situation, but she is older and they did not resolve it

Your relative is going to have to request the unlock code if the meet the requirements.

Tmobile will only unlock phone for tmobile customers.