I'm having this problem with Textnow (otherwise great app), where I can't receive any calls while I'm on cellular data. I'm using a data-only plan with Public Mobile in Canada (runs on Telus towers), and I can make crystal-clear calls so the network speed shouldn't be a problem. However, whenever someone calls me, it would not ring on my side. After a few minutes, I would get a notification saying I missed a call. Same with texts - a long delay before receiving. It works flawlessly on WiFi and no problem with outgoing calls on data. I tried clearing cache and data, as well as reinstalling, to no avail.
I'm using Huawei Mate S, not sure if it's an optimization thing. I received no response from the customer support through ticket, I really hope this gets solved soon. Thanks!