Is one Apple account being shared across multiple phones? Each phone user should be using their own Apple ID for iCloud, iMessage, Apps, and so on. Please make sure of this.
Sing in with your Apple ID on both phones. On the phone app thru "settings" (opening the "settings" app first, then tapping on the "phone" icon) of your main phone, select or make sure that "call on other devices" is set to "when nearby" or "always" (if the latter is available). Do the same on the other phone if that option is available.
Wait a minute. Do you want to turn it off? Then you can still do that with similar instructions that I gave you above. Open the "Settings" app; scroll down to "phone," tap it; tap on "Calls on other devices"; Change that, and you're done. Do that on both phones to make sure.