|There can be acceptable reasons for an interviewer to take a phone call during a job interview, but your interviewer sounds both cordless and clueless. If an interviewer absolutely must take a call, it is a combination of courtesy and businesslike behavior to mention the possibility of such a call at the outset, and to apologize for the interruption afterwards.|
One of the most basic rules of interviewing is to keep the sessions free of interruptions if at all possible. Not only do interruptions undercut the flow of the discussion and cause needless backtracking, they also send a subtle message that the applicant is not particularly important.
As for the best action to take in the event that another interviewer decides to talk on the phone rather than talk with you, your present strategy seems to be the way to go. There is no point in lecturing the interviewer, as you can easily end up looking like the one with the problem, and, ultimately, the one without the job offer.
Remember that companies send many obvious as well as subtle messages about their values, culture, and climate during the pre-employment process, and some of these messages even come by telephone.