@EricaGyarmati
I would suggest that instead of testing the SIM card in another phone, you should try testing another SIM card in your phone. (Or do both, if you like, wouldn't hurt.) The problem is your phone or the problem is your network and changing your SIM on the same network you can already connect to isn't going to change anything relevant.
It's possible your phone has bad radio. Intermittently disconnected or misaligned antenna, malfunctioning component, a single-bit error in firmware causing problems. Bad speaker is not an uncommon problem. Bad microphone as well, although that would be on another phone (the person you're talking to) instead of your own. If the phone has ever been dropped or had any liquid get inside then these (and other) hardware problems are far more likely.
The other possibility is that there simply isn't good radio coverage from Telus/Bell/SaskTel/etc networks in the area you use your phone. Low signal "bars". Consistently weak battery. Interference from certain phone cases, automotive devices, home appliances, industrial fixtures, etc. Public Mobile (Telus) (and Bell/etc) uses HD Voice and VoLTE protocols - they are higher-quality and guaranteed-bandwidth communications modes with some redundant encoding and embedded error-correction - they certainly don't provide "audiophile" Hi-Fi sound experience but (when there aren't bad conditions) they can approach CD quality audio on voice segments of the sound spectrum.
A final possibility - maybe - is your iPhone software settings. iDon't iPhone so iDon't really know ... but you might get different results if you do a network reset, switch "3G" and "4G" modes, select operator automatically (Auto) or manually (Public Mobile), etc. Misconfigured software causes the overwhelming majority of problems reported by consumer devices. Most Android phones have volume sliders for mic sensitivity and speaker amplification - or can install apps which provide those controls - so I'd be surprised it iOS counterparts can't do the same.
If you use hearing aids then you'll probably benefit from T-Coil settings or custom app controls.
