Straight Talk is a cell phone carrier, I’m going to need a little more information regarding the “fixed Cordless” aspect of you question. Straight Talk is requiring that customers upgrade to a Cell phone that is capable of VoLTE by no specified date.
If you are interested in connecting your cell phone to a cordless phone for use in your home, you can take a look at what Panasonic has to offer. These cordless phones are capable of connecting to your cell phone via Bluetooth.
You can find a list of compatible VoLTE phones here.
I hope that helps you out