The telephone was not able to answer by an Auditee after continuous rings because the she was attending to the needs of internal audit teams that resulted in customer complaint.
If you only have one person who is available to answer a phone, you have a bigger problem! This has nothing to do with the fact they were being audited! What if they weren't at their desk - for a bathroom break, for example? (they are allowed to do that I hope). That would also result in a problem wouldn't it? Out sick unexpectedly? does your switchboard operator know who to direct the call to, if it doesn't go through to this key person. You must have 'plan B' for the operator to take a message etc. I know my company does!
Good, customer focused, companies don't rely on automated systems to answer the phone. They are nasty and frustrating. Customers call to speak with some one, to ask a question, get advise or similar. We don't want to listen to a machine! Out of hours, we expect a machine, but not during working hours.
Internal audit etiquette doesn't have any relevance here, although the points made are valid, in reality.