How secure, are mobile SIM communications?

It’s very common to hear about encryption in web browsers and apps, but that’s all about the internet. But what about mobile communications that use a SIM card, like ordinary text messages and voice calls, that does not require internet access. How are they encrypted ? I’ve heard of protocols like 3GPP, and many others, but I dont think they are mandatory!
Every help and guidance will be appreciated!!!

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Author: Vivekanand V