Be on the phone
Definition 1 of 1
Pro Tip 1/3Polite Interruptions
Use the phrase to politely interrupt or inform others you can't speak because you're busy on a call. 
Can we talk later? I'm on the phone right now.
Pro Tip 2/3Cultural Norms
In many cultures, it's considered rude to be on the phone while interacting face-to-face with others. 
Sorry, let me step outside. I'm on the phone with my boss.
Pro Tip 3/3Call Duration
The phrase doesn't specify how long you'll be on the phone, just that you are currently engaged. 
I can't help you at the moment; I've been on the phone for a while.