Is there a way that I can set my phone so that I can receive calls at all times

To make sure you receive all calls, go through this list.

First, make sure Do Not Disturb is disabled. Go to:

Settings -> Do Not Disturb. Turn Do Not Disturb OFF.

then, go to:

Settings -> Sound & Haptics.

Make sure your ringtone is turned up completely and choose a ringtone that you are sure you can hear. Also make sure "Switch with Buttons" is turned OFF. If that function is turned on, you can turn the ringtone up and down with the Volume buttons - you are not interested in that, because you can often turn down the volume completely without thinking about it.

then, go to:

Settings -> Phone -> Forward Call

Make sure Call Forwarding is turned OFF

then go, one step back to:

Settings -> Phone -> Blocked Contacts

Make sure there is no one on the list. This list can be used if you receive unwanted calls, eg from a seller. If the list is empty, then you are sure that all calls go through.

Then, on the side of your iPhone above the Volume Buttons there is a "Switch". This Switch can switch from sound to silent. If you look into the switch, it will be red if the phone is silent. Then it is red then switch it to the side. You will also receive a notification on the display that Silent is OFF.

Finally. You can also choose how long your phone should ring before it is forwarded to Voice Mail, or just reject. This feature is a Carrier option, so there I would suggest calling your Carrier and asking them how to order the phone to call as long as possible. Most often it is just a code you have to press inside the Phone app - on the phone keypad. You know * # 5001 like.

These are the things that can prevent you from receiving or discover calls.

By default, your iPhone can always receive calls. Only you can set to NOT receive calls, for example. by enabling Do Not Disturb, or forward all your calls directly to VoiceMail.

Are you not receiving all your calls now?

Go to settings, do not disturb and make sure that is off. Go to settings, phone, make sure silence unknown callers is off. You should then get calls all the time.