How to get values from a table without duplicate values

I have a table like this :

id -  phones - group_id

0  ---   11------- 1

1  ---   22------- 2

2  ---   12------- 1

3  ---   11------- 1

4  ---   45------- 1

5  ---   44--------1

Now, How can I get these values ?

11,12,45,44

for example : ".... where group_id=1"

I Google it can't find exact solution.