Welcome to the community!
It sounds like your phone is not receiving signal at all times. I think the messages you receive following missed calls comes from Fido's voicemail notification service. I'm not sure why the American Flag shows, though. I, too, received one of those messages to notify me that there was a voicemail left. I think it might have been when Fido switched to a new voicemail system, perhaps?? I usually get voicemail-to-text notifications, but that day I received both the American Flag one as well as the text transcript. I now only receive the transcript version which appears to come from the caller's phone number.
@FidoSolutions Is the voicemail notification system showing the American Flag for some reason? Unfortunately, I don't have the message anymore to create a screen capture.
What signal strength does your phone show? Is your phone capable of voLTE? If not, you might consider disabling LTE on your device. You can do so by setting your network selection to GSM/WCDMA(UMTS) or 2G/3G. With the exception of voLTE and Wifi-calling, voice calls are transmitted via the '3G' networks. It's possible that there is a delay in your phone switching networks causing your phone to miss calls.
On the other hand, if your phone is supported for Wifi-calling (and has access to Wifi), you might consider trying that feature to see if it might help improve your call situation.
Are you able to test your SIM card in a different device? Simply re-inserting the SIM might also help as well.
Hope this helps 😀