Stored speed dial numbers can be called on the Samsung device by tapping and holding the respective digit on the dial keys. For speed dial numbers of ten or more (i.e. two digits), the last digit must be tapped and held down.

Example speed dial 145: Type 1 and 4 as usual as phone number, then type and hold 5 to start the call for speed dial 145.

Android 10 (One UI 2.0)
Step 1:  Open the  Phone App
Step 2:  Tap and hold the  Speed dial number
  1. Open the Phone App
  2. Tap and hold the Speed dial number

Samsung Instructions