If you have an iPhone, you can use the "Do Not Disturb" feature to silence calls from unknown or unwanted contacts. While not setting a custom ringtone per se, this approach can be helpful:
Go to "Settings" > "Do Not Disturb."
Enable "Allow Calls From" and choose "All Contacts."
Calls from non-contacts or blocked numbers will be silenced.
Dedicated Call Screening Apps
There are apps designed specifically for call screening and customization. Apps like "Truecaller" for Android and iOS provide features to set custom ringtones for spam or unknown callers.
Using Ringtone Maker Apps
You can create custom ringtones for specific contacts using ringtone maker apps. These apps allow you to trim audio files and set them as ringtones for individual contacts.
Assigning Silent or Vibration Patterns
For contacts that you'd like to keep discreet, you can assign silent or customized vibration patterns. Both Android and iOS allow you to set unique vibration patterns for specific contacts, helping you differentiate calls without a sound.