Finch didn't want The Machine to talk, which is why he took away its speech, didn't want it to communicate, which is why he force it to use only public phone and to use Dewey Decimal System(DDS) to communicate the SSN
Finch says he saw every line of code when he create it, but some how
was able to alter those codes, change its DNA, that's why The Machine talks to
Why does The Machine talks to this person only, when Finch clearly made sure that It wouldn't communicate with anyone, not even him? Did the person modified its code or did the machine modified its own code?
said that The Machine respects Finch and the choices he made. Which may be the reason why The Machine wouldn't talks to Finch when it could talk others.