How was the signal reception in your area? did you have a full bar of service there? Also, have you tried to reset network settings of the phone and see if it can do anything? Did it happen to only one place or everywhere you went to?

To check if that was an hardware issue, do you have another “good” phone around to put the SIM on and check and see if the problem follows?

It happens anywhere I am. I do have another phone (Galaxy) and it works fine. My bars range from full to one bar less than full. I will find out how to reset the network settings and see if that helps. My SIM was originally in the Galaxy with no issues, just started with this new phone. Not right away, either, just only started happening the last couple of months but now is getting worse by the day. 

P.S. thank you for your help!

I would suggest you try to put the SIM back into the Galaxy and test at the same location. We need to identify if the issue was from the hardware or not (so you can ask them to repair). Sometimes the problem was from the network in the area (depending indoor or outdoor situations). The cellular signal is generally getting worse if the calls were made indoor and there were plenty of partitions around. 

Okay, I will do that and test it for a few days, to be sure. Thank you!

To reset your Pixel 4a 5G’s network settings, the video in the link below provides step-by-step instructions.

How to Reset Network Settings on Google Pixel 4A

I tested for a month and the problem transferred to the other phone. So it's not the actual phone. Can it be the SIM card?

Have you tried putting your Pixel 4a 5G into Safe Mode?

In this mode, your Pixel 4a 5G will only have access to the apps that were pre-installed from the factory. When the phone is in this mode, try making a few test calls or listening to messages. If the issue disappears while in Safe Mode, this means that the phone’s operating system is intact and that a third-party app is likely causing some sort of conflict. This also rules out a problem with the SIM card.

If the problem persists when making calls or listening to messages while in Safe Mode, then you may have a SIM card that is showing the early signs of imminent failure. I would suggest that you head to a Koodo kiosk or an authorized TELUS/Koodo seller to have the SIM card tested or replaced.   

To activate Safe Mode, hold the power button until the Power off icon appears. Press and hold the Power off icon until the Safe Mode prompt appears. Select Safe Mode to confirm. Once your device has powered on, you should see the words “Safe Mode” in the lower right or left corner of your screen.

To exit Safe Mode, press and hold the power button and select Restart. If you are not prompted to restart, continue to hold the power button until your device restarts.