edit /etc/resolv.conf
nano /etc/resolv.conf
make your changes. Then CTRL+O, Enter to save the file the same name. CTRL+X to exit from nano
File should look like:
search pbx.something
nameserver 1.1.1.1
nameserver 8.8.8.8
substitute your name server addresses for the 1.1.1.1 and 8.8.8.8