Random Missed Calls

On a very inconsistent basis, some of our users will receive notifications for missed calls without their phone ever ringing. We've confirmed that the calling device will ring for 30 seconds or so and then go to voicemail, but the end user will never receive the call, just the notification that they missed the call. Our current solution is to turn off Wi-Fi calling, but our office building has poor reception and therefore isn't a great solution.

For additional context, all user are using iPhone on company assigned business lines. Just curious if anyone else has ever experienced this issue and found a solution.

Thanks!