I have great difficulty in understanding the Scots and the Geordies
A sweeping statement if ever I saw one. I am a Scot, but have lived over half of my life out of Scotland. I am sure you could understand me
It is about pronunciation. Communication is the watchword and if you do not understand the message being communicated to you, even if the person speaks grammatically correct English, then it it is down to among other things, the pronunciation. Are you really saying you do not understand an air traffic controller simply because he is Scottish? He could be using the exact same terminology as a Nigerian air traffic controller and both would be expected to use a similar level of diction and pronunciation to be understood.
It seems absurd that having just had a telephone conversation with someone (I assume to be) a native speaker, any further test is required
The person on the receiving end of your call cannot verify who you are. You could be a friend acting on someone's behalf. And before you poo poo the idea, a West African Brit was able to join the Immigration Service some years ago by having a third party attend the interview and the subsequent training course. It was only after he was found not to be up to the job that he had used an imposter to get him the job.