I recently bought an iPhone 6S, but my trusty iPhone 5S is still in good condition so I figured I would keep it around for use at home. I'm currently using the iPhone 5S as "fixed line replacement" with a VoIP app, as an Apple TV remote, and some other purposes (basically like an iPod Touch), so there is no SIM card in the phone and it's in Airplane Mode with WiFi on (I'm thinking of putting an inactive SIM in there to get rid of a few error messages though).
Recently I noticed that when someone calls me on my iPhone 6S, after a few seconds the call also comes through on the iPhone 5S thanks to Continuity, which is very handy. The thing is, it doesn't work in the other direction seeing as this is an actual iPhone, it will complain (both in Messaging and Phone apps) that either there is no SIM installed, or that Airplane Mode should be disabled.
Is there a way that I can get the iPhone 5S (without SIM, in Airplane Mode with WiFi on) to behave like an iPad and use the cellular connectivity of my iPhone 6S via Continuity over WiFi, so that I can also make cellular calls and send/receive SMS/MMS messages on the 5S (WiFi only) when the iPhone 6S is nearby? Or are the incoming calls really the only possibility in this case?