Substring

Hello;

I have an sql script function that take one text parameter "funct (text)", what I need to do is the following:

If the parameter name is string and its value was for example "abcd" then I need to do a query based on ab and then
basedon the abc, how?
 

Example:

SELECT * from voipdb where prefix like string

But I need the string to be ab and then to be abc? How I can assign the string to the first character and then to the
firstand second? In other words, how can I can take part of the string to do query on it?
 

Regards
Bilal