Since 3G is being shutdown by local networks by end of 2025, this is a reminder to check your phones for compatibility with the 4G/5G standards.

Older phones, phone with old firmwares, 3G-only phones and some imported phones may not comply with the requirements for a 4G/5G-only world. 

The phone needs to support the VoLTE standard to be able to make phone calls. 

If you noticed that the network icon on your phone switches from 4G/5G to 3G when placing a voice call, then you need to check that your phone supports 4G/5G networks, is set to use 4G/5G or have updates available.

Another way to check is by sending a message to 550, cointaining "3G" (without the quotes). You will receive a reply with its current status.

3G-only phones won't connect to any network after the 3G shutdown. 4G phones without VoLTE support will not be able to make voice calls (although it may still be used for data-only).

You can find more information here: https://www.tcf.org.nz/digital-living/understanding-the-3g-shutdown