Student or Learner
Someone called me at appointed time. May I reply as below:
I am finishing something. Can I call you in 15 minutes?
I find the reply a little terse. I would probably say (especially on the phone), "Sorry, I know I said to call me now but I'm just finishing up something here. Can I give you a call back in 15 minutes?"
"I am finishing something" just sounds rather abrupt and formal.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.