I see that you are having trouble with placing/receiving calls. I'm very sorry to hear that you are having trouble with T-Mobile and your Samsung Galaxy S8+ device. I personally have a S8 and would love to help you as much as possible. Please keep in mind that I am not a T-Mobile rep, so if you need their help, please call 611 from your T-Mobile phone, or contact them using their social media accounts, or use myT-Mobile to initiate a chat. Please remember to not post sensitive information on this forum.
First of all, Is your device able to connect to T-Mobile's 4G LTE network at where you are having these call problems? If you cannot connect to ANY network at all, you may be outside of a T-Mobile coverage area. Please check here if your address is within coverage area. Also, it may sound very dumb, but sometimes turning your device off then on helps!
If it turns out you are just having poor coverage at your residence area, here's an article I wrote about signal boosters, which are small cell towers to put in your house that will dramatically increase the coverage of the 4G LTE and call networks. You can order one from you local T-Mobile store or from 611.
Could you please confirm with me that your phone was purchased from T-Mobile, and is fully compatible with the T-Mobile network? If you are a Bring-Your-Own-Device customer, please check here using the IMEI number of the device to see if it is compatible. Just go to your dialer, and dial *#06# and two barcodes should show up. The top line is the IMEI.
Please take the time to try these steps and let me know the outcomes so we can move forward from here.