Many approaches. Among most popular:
Multi-tap
Press key multiple times to reach desired letter
3 Χ c + wait + a
+ t = cat
Tegic T1
Use frequency of English words to place most likely
alternatives first
Use a next key to indicate
next alternative
2 Χ
ba + act + next = cat
Common feature:
Use one key for space (e.g., 0), another for symbols (e.g., 1), so less
than 12 keys