Zello.
https://zello.com/app
I used this when I was running the Pentastar Dusturd. It's easily the least-worse cheap solution. The app essentially turns your phone into one of those old annoying Nextel Walkie-Talkies. The best part of this approach is that anyone with a cell phone can contact the driver at any time. If somebody didn't catch the latest message, they can go through the queue of received messages and replay any that they didn't hear correctly (instead of going back and forward: 'what was that again?!')
A message will only be sent / received when the device has adequate cell service. If service isn't available, it will be stored in a local queue until a signal is obtained so it can transmit the message. In reality, there's no limit to the range. I had guys in Korea following our team in the 24 hour race in Chicago last year. Be warned though, they could have been assholes and started chatting up a bunch of bullshit. It's best to make a private invite-only chatroom to keep the riff-raff out. The only down side to this, is that you're at the mercy of your phone and when (not if) it decides to reboot itself and / or randomly force close the app.
Also, you need to get creative with making headset adapters from the stock 3.5mm mini headphone jack in the phone. I eventually bought a splitter and wired a push-to-talk button. Unfortunately, your standard phone push to talk button shorts the micro phone connection to ground meaning you can't talk into the microphone while the button is pressed. What you CAN do, though, is map this button in the app to "toggle" the transmission. Push and release, start talking, push and release again to stop talking and send the message. Of course, there's a lot of potential for less intelligent drivers to not press the button a second time and continue to record a 3 - 4 minute (it eventually times out) message.
When everything is working properly, it's a great system. However, shit goes south real fast when one little thing stops working correctly. Using a Rugged Radios setup and IMSA headsets with the latest team (Sheen) and probably won't go back to Zello. The simplicity of a dedicated radio system can't be over appreciated.
The Pentastar whisperer