I can't recall when this started happing, but it was probably over a year or two ago.
When the phone screen is locked, incoming calls would get one right, and then straight to voicemail.
If the phone is unlocked or if the caller calls back a second time after the first attempt fails, there is no problem.
I've tried to use Fongo on several devices such as Xiaomi and Samsung.
Currently using the Galaxy S24 Ultra on Android 14 and the issue is still there.
I don't want to enable Wi-Fi lock as it will drain the battery, and it doesn't seem like any other apps like Textnow, Whatsapp, Facebook Messenger have issues when the screen is locked.
Any ideas on how to fix this?