Could you phone me back in 10 minutes, please? I am not able to speak right now.

if you wanna use the word convenient than the better construction would be: " IT'S NOT CONVENIENT FOR ME TO SPEAK RIGHT NOW" . But deffinetly "I'm not able to speak right now" is simpler and better. peace out

Thank you very much Angela!

"I'm sorry, I can't talk right now. Could you please phone me in 10 minutes?" "Speak" just sounds wrong here, unless you're physically unable to speak (your voice has stopped working). In this situation where it is a matter of convenience, I would use "talk."

Thank you Manar!