Thanks a bunch! I haven't tested but hopefully it would work better 
But I still don't really understand how we change the frequency (a tone or pitch if you will) of a DAC? Now we have increasing-decreasing tone (triange?). How to keep in the same frequency?
As I read through more forums I kinda understood that we can use DAC like that:
analogWrite(DAC0, x);
//x = 0 - 255
Where x would be a different voltage sent to DAC. Different voltage = different frequancy?
But when trying the line above - nothing can be heard 
I need "a humming sound" for background when picking up the phone, which is also pretty difficult to achieve 