Hi there, Pete,
Currently, the Favorite contacts feature is the only way to use any type of speed or quick dial function on your iPhone, as detailed in the Apple Support article Make a call on iPhone - Apple Support:
Call your favorites
Tap Favorites, then choose one to make a call.
On models with Dual SIM, iPhone chooses the line for the call in the following order:
• The preferred line for this contact (if set)
• The line used for the last call to or from this contact
• The default voice line
To manage your Favorites list, do any of the following:
• Add a favorite: Tap
, then choose a contact.
• Rearrange or delete favorites: Tap Edit.
When you press and hold a number on the keypad, you get an option to add it to your Contact list, either as a new contact or as an additional number for an existing contact. It does not assign it as a speed dial, however.
If you'd like to see a change to this in future iOS releases, please let Apple know directly here: Feedback - iPhone - Apple
Thanks for visiting the Apple Support Communities.
Have a wonderful day!