This app was mentioned in 7 comments, with an average of 1.14 upvotes
If both are connected to the same wifi:
Talkie - texts, calls, file transfer for smartphones
KDE Connect - file transfer, clipboard share etc. for smartphone or PC
First of all don't be dumb and use any unencrypted form of call or message. You would be particularly dumb of you use call or SMS.
But most importantly, try to avoid using cellphone.
You can use this for short range walkie talkie function over hotspot: https://play.google.com/store/apps/details?id=com.remaller.android.wifitalkie_lite
Come to think of it, most US carrier phones have locked mobile hotspot. You people are dumb and don't demand enough freedom.
Edit: Sorry I tried to find the Fdroid apk, but couldn't. If someone does, please link.
https://play.google.com/store/apps/details?id=com.remaller.android.wifitalkie_lite&hl=en
Other than this wi-fi walkie talkie app, you can get a google voice number for free and then use Talkatone
There's Talkie. No idea how reputable it is or if it even works but there you go.