2 way audio - push to talk from 1 phone not working iPhone 6

Hi there.
I just set up a Birdfy for my parents on their iPhone 6S plus but when I try to click the Mic to talk (it is allowed to use NetVue in the settings) it shows the mic ---------- line but when I talk it doesn’t work.
The microphone definitely works on other apps on that phone.
When I connected via my iPhone X it worked fine … same settings.
Could you please look into that? It’s not urgent but would be nice if they could use the 2-way speaker.
Many thanks in advance,

Hi, Gene,
Please check if the APP version on the iPhone 6s is the newest 5.23.6.
Kindly try to uninstall the Netvue APP and then install it again.
Let me know if it works or not.