Hello.
Have several old perfectly working decommissioned 'Android smart phones' without service provider.
Would like to have 3 with a walkie-talkie application on Wifi or Bluetooth without any home router nor internet in play. Wifi DIRECT one device to the other as PTT intercom, or full duplex.
Get confused at time to choose which application to install as seems everyone assumes everyone has Wifi at home and everyone is an expert in handling wireless configurations. Am the flag-bearer of networking idiots and cannot deal with it. Besides cannot discern if the application to choose is or not operation-dependent on an undesired home router.

What do you suggest about solving my hurdle ? I can install from a Wifi router for accessing the 'play store' from the phones. Any keywords I should look for direct Wifi ?
The last app I tried was "Lucky Talk" which did not work and when tried, said "10 minute free trial" Another demanded to have Android 10...

 

I use WhatsApp.

 

Can’t you make one of the phones a hot spot and connect the other two to it? Then you have a WIFI network with the first phone being the router.

 

You can use BlueFi Phone (Bluetooth & Wi-Fi Walkie Talkie).

 

BlueFi... Hot spot... And how do you make it work without a cellular provider ?

 

I don't think you'll find COTS for this, has to be coded into APK.
I think the wifi network is the easiest, the app can then use IGMP to join a multicast group. Transmit pushes data to the mcast address and Receivers ingest that data. A Transmit would mute Receive function, just like WT's do.

A ddg search told me this for a old phone PTT solution:
RadioPro Talk over cellular and wifi

But the range for phone wifi is probably way less than the $10 WT set from walmart.

 

Digital packet over twisted pair and wireless telecom should not be defined in terms of PTT
The software development role is too dominant. 1973 bell labs Michael Lesk program called "Talk"
Packet, IRC chat, Text messaging are but a few spin offs. Voip uses many lines of code.
The analog to digital can use a usb sound card, It is this scenario that could be ported wireless short range.
This has the least radio emission but a hard sell due to the momentum in connectivity objectives.
In a network landscape there can be re-purposed cell phone blue tooth to network repeater if it meets all the protocols.

 

I think there are two issues here. First, you need a Wi-Fi connection between the phones. Wi-Fi Direct or some kind of mesh might be an option if the phones allow you to configure it. If not, the most straightforward option is probably to set up an access point, either on one of the phones (as previously suggested) or with a standalone AP. Make sure that client isolation is turned off. Having an access point doesn't necessarily mean that it provides Internet backhaul; it is just a way to get all of the phones on the same LAN segment.

Second, you need the app running on the phones to either use broadcast/multicast or to send separate VoIP streams to each of the other phones. Most VoIP apps are not set up to do that; they relay through a server in the cloud somewhere.

If you sometimes have Internet connectivity and want to utilize it to connect to distant devices at those times, take a look at ZeroTier or other similar networking software. They make it look like your devices are all on the same LAN even when they are not (like a VPN) while routing data directly peer-to-peer in most cases.