Can an iPhone without a SIM card receive phone calls or call other people?

I received a call from my mom and my phone didn't have a SIM card?

Yes

It can only call either 911 or use a Wi-Fi calling app to call other numbers

You would only be able to get VoIP calls through apps like WhatsApp, Facebook Messenger & FaceTime.

Without a SIM you can't get carrier based calls of course since the number that is assigned to the SIM isn't active.