Well first you're talking about two different things. GSM, CDMA, LTE (global) are forms of data and do not affect your cell signal. So making a change there would benefit you if you were having problems sending emails or getting online for a game or updating social media but not for calls. There are any number of reasons calls are being redirected to voicemail with the simplest being that you have a poor signal wherever you are when they try to call. The next simple issue would be your phone radio is not picking up a good signal. We often hear, with my old phone XYZ I got great service at the office but with this new phone QWERTY I don't. The cell signal in the area is still the same but the new phone had a different radio set and doesn't get the same reception.
Moving on, there could be cell work going on in your area. Mobile companies are constantly servicing, adding, upgrading towers without notice. Sometimes they will provide information online but you'll usually have to go looking for it.
Have you checked your do not disturb settings? Depending on your device DND is often programmable and can come on based on a schedule. Samsung's Bixby can run routines as well blocking calls in certain locations or at times, I think it uses DND but I'm not sure and I'm also not sure what device you have so there could be half a dozen other reasons for this issue besides these.
If you'd like to reply to this thread and provide more details about your issue, including;
What device you have?
Are you in the same location around the same time of day when you are missing these calls?
Are the missed calls always from the same people?
Do you get the voicemail notification right away?
Are you sure the volume is set correctly?
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https://forums.androidcentral.com/showthread.php?t=409154