Open the Phone app
Tap Keypad
Tap the number keys that correspond to the letters you want
Use the standard phone letter mapping:
2 = ABC
3 = DEF
4 = GHI
5 = JKL
6 = MNO
7 = PQRS
8 = TUV
9 = WXYZ
For example, 1-800-FLOWERS becomes 1-800-356-9377
If needed, type the letters in Contacts or Notes first, then convert them using the keypad mapping