Call Hold/Call Wait allows you to place the call on hold and answer another call. All the latest model of mobile handsets already has this feature pre installed. All you have to do is enable it from your mobile phone.
For Android Phones (Samsung Galaxy S10)
A. On your Home screen, tap the Phone Icon and click on the Menu button
B. Select Settings and go to Supplementary services (or More Settings on lower Android version)
C. Click Call Waiting to enable and you're all set!
For iPhone (iPhone 8)
A. Go to Settings and select Phone
B. Look for Call Waiting and tap on the switch to turn ON