If you want to allow a contact to ring on silent, there are a few things you can do. One option is to go into the Settings app and tap 'Do Not Disturb'. From here, you can tap 'Allow Calls From' and then select the contact or contacts that you want to be able to reach you even when Do Not Disturb is enabled.
Another way to allow a contact to ring through even when your phone is on silent is by using the VIP feature in iOS. To do this, go into the Contacts app and find the person you want to add as a VIP. Tap on their name, scroll down, and tap 'Add to VIP'. Now, whenever they call or text you, their message will come through regardless of whether or not your phone is set to silent mode.
Of course, if you just want a specific call from a specific person to come through even when your phone is set to silent, you can always just turn off Do Not Disturb for that one call. When thecall comes in, long-press on the volume icon in the top left corner of your screen and tap 'I Want To Hear This'. Thecall will come through loud and clear!