Is it correct to say:
Hi I'm calling "concerning" something....
to tell someone that I'm calling to discuss something or to talk about something...
Is it the same to say:
Hi I'm calling you "regarding" something...

Question

Is it correct to say:
Hi I'm calling "concerning" something....
 to tell someone that I'm calling to discuss something or to talk about something...
Is it the same to say:
Hi I'm calling you "regarding" something...

Answer

Yes, they can be used for the same thing.
Just to note though - concerning something sounds a bit more serious.  Regarding something sounds  a little less serious but still formal.

Answer

yes, 'about' is most natural but 'regarding' sounds more official.

I wouldn't say 'I'm calling you concerning'.., no.
:)