There is so many Walkie Talkie APPs all around the web, that are using WiFi or Bluetooth to transferring the data (at least all the ones I've ever seen) and none is using inner antenna to transmit data over radio waves as a real Walkie Talkie device.
Is there any security reason for it? or is it restricted to access the antenna as a sender/receiver?
The thing you are looking for is peer-to-peer connection using mobile networks. It is impossible due to the radio services the mobile networks are licensed under vary by country as stated here.
Technically as said here You need a cellular network provider to communicate between two devices.
If you still wanna make a P2P communication Use Wi-Fi peer-to-peer (P2P) API Follow https://developer.android.com/training/connect-devices-wirelessly/wifi-direct