Spudster already mentioned this (however, it wasn’t acknowledged if you have an iPhone), but it does have this feature that silences unknown callers and sends them straight to voicemail.
Settings > Phone > Silence Unknown Callers > Off
I’ve also noticed with my iPhone that when certain centres call me, especially medical centres or hospitals, it’ll ring once (or not at all) and go straight to voicemail. However, the person calling me swears it rang a handful of times before going to voicemail (but I only hear it once).
I do hope you get it sussed out soon.
@chromeknickersSince Public Mobile does not support VoLTE, when a call comes in the phone has to switch down to 3G mode to take or make a call. If that transition takes too long or fails, it could result in the symptoms you describe.
To work around this potential cause, you can try disabling LTE by forcing the phone to 3G/WCDMA/HSPA mode so it doesn't have to switch networks to start a call.
Curious to hear results of forcing to 3G. Maybe you have weak 3G coverage in some areas, and the LTE to 3G transition fails occasionally.