When focusing on a specific task, interrupts might popup when you least expect them or need them. it is not wise to respond to every phone call – you have your own deadline; your agenda is vital for your success. Unless it’s your family or your boss, such calls might drag you away from your purpose and fulfilment of your duty - depending on the nature of your work.
When someone is looking for you while you were unable to call them back they expect you to all the back, this will allow you to keep a good working relationship with them.
Keep a special space just left of your screen for such reminders and write down the name of the person who was looking for you, then get back to your work. Having such a list will instantly remind you to whom you need to call back when the current cycle is over.