My suggestion is to make sure you define with your manager when you are taking lunch and that you will be at your desk to make some calls for an impending event. If that is not an option then leave the office for lunch and make calls from your cell phone.
The importance of talking to your manager is that they will know if they walk by and hear you what is going on. Also, if the co-worker in the cube next to you overhears and feels compelled to let your manager know then the manager can explain.
Most managers know without even telling them that you would not abuse your time but it never hurts to explain. When a major life event is occurring and it will cut into your work time you want to make sure you go on the record about what is happening.
Let me know how you have handled a major life event and work at the same time.