Based on what Stewart1 is saying, it might help to know what model of Grandstream Gateway you’re dealing with. When you say POTS line, is this a line going to an analog phone that you want to set up as an extension, or is it a telco line that you’d like to place calls out over to a specific number?
For example, do you want to set up a Ring Group where whenever ext301 is called, it will ring a sip phone, and also call an outside number over a POTS telco line? If this is the case, and you’re setting up a ring group, you can add both extensions and external numbers to the Extension List box. Similar to what’s described here: [SOLVED] How would I properly add my cell phone number as an extension on my PBX?