just remove the 'because' from each sentence and they are fine.

I called to say THAT I can't come. OR I called because I can't come. I called to say THAT I won't be able to come. OR I called because I won't be able to come. I called to say THAT I am not going to make it. OR I called because I am not going to make it. I called to say THAT I'm not coming. OR I called because I'm not coming. You cannot use "I called" followed by BOTH "to say"" AND ""because" Use one or the other, but NOT BOTH.