Apple on iOS has not changed the look of the phone app’s keypad look for a very long time. If you are bored with the same look and want to customize it then there is a Cydia tweak called “NPF” by iOS developer Easy-Z where you can basically change the shape and size keypad button to give them a new look.
This Tweak is a re-write version of the popular Cydia tweak called Callor and works with latest Jailbroken devices.
There are few settings where you can customize the look of the phone app’s keypad look from the preference pane that is available in the Settings app of your iPhone.
The tweak is available to download for free on iOS 11 and iOS 12 Jailbroken devices for free from https://repo.dynastic.co/ repo.