Extreme delays in voicemail notification on lockscreen

I've had this issue since soon after I bought my iPhone 6s from Xfinity Mobile about 2 years ago.

Namely, when the phone is on lock screen (whether manually or automatically initiated), the voicemail audio alert (my main concern) and visual notification tend to be extremely tardy.  The delay is quite variable, but can be anywhere from 10 minutes to 10 hours.  1-5 hours delay is typical.  If, during that delay, the phone comes out of sleep for any other reason -- whether getting a text notification (*those* always work fine), a call coming in, or simply my picking up the phone -- the vm alert will immediately sound/appear.

One oddity I should note: When the phone is plugged in (i.e., charging), the vm audio alert and screen notifications always come promptly, within a few minutes of the voicemail being recorded.  Keeping it plugged in is obviously not a practical solution, but does this behavior help point to possible underlying issue(s) that can be addressed??  I would really like to avoid doing a full reset (and I doubt that would help, anyway).

Any suggestions?  The OS is up to date.