more likely, that person mis-type their own number as yours by mistake

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Agree. Recently, my 4G non-VoLTE phone on Postpaid GX50 could not complete calls on 4G = had to switch to WCDMA/GSM or non-4G to call out or to make calls or for the call to complete. Likely the same if want to receive calls.

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Maybe your callers are using non-VoLTE phones.

Since, my problem happened only recently, it should have something to do with the setting of U Mobile's 4G/3G cell towers.

Eg before my calling problem started recently, my non-VoLTE 4G phone could not surf the Internet or use data when  on 3G or WCDMA but after that can surf, albeit much slower than 4G = U Mobile has recently changed the setting for 3G on it's cell towers.
....... Note that U Mobile is also riding on Celcom's 2G, eg for Emergency/999 calls, SMS and OTPs from banks, online shops, etc for non-VoLTE phones.
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My apologies. My above calling problems is "user problem", ie my "Contacts" app phone number which I called did not include the country code "+6" for Malaysia, ie I entered only "011-XXXXXXX".
I have just tested on my non-VoLTE 4G Android phone by calling out, with the Mobile Network set to GSM/2G = no problems.