This is impossible to answer, as You have given us absolutely nothing to go on!

What do these deviecs actually do?
How are they supposed to work?
What do you mean by "combine"
What is "the RS232" supposed to be doing?
What do you mean by "isn't working"
What steps have you taken so far to determine what's happening? What were the results?
Where does DTMF come into it?

and so on...