Hi,
It is interesting you are concerned about the call. Your agent is correct in the fact the test is not designed to "catch you out" but provide an opportunity for you to demonstrate your knowledge. I noticed you did not mention what visa you were applying for but anyway, here are some of the things they will talk about based in my experience.
1) They will go through each job you have had and ask you to discuss each role, duties, who you worked builds completed etc... As part of this they will be able to gauge the basis of your skills and what you have been doing.
2) They will talk about preparation of dust, sand, work area, health and safety and all elements to ensure you will be safe to yourself and to others in your approach to your job.
3) Technical aspects will include Arches, Corners, pillars and causal & effect questions. i.e. If the dust used was too Dry, wet, bad mix, what bricks are right for a job and anything that could harm or improve structural integrity.
The most important thing to remember is not to BS and if you do not understand a question ask them to repeat it in a different way or simply say "I don't know." That is far better than giving the wrong answer just for the sake of talking through nerves and relax, enjoy yourself and smile when you can.
One final thing - do NOT be tempted to tell them you are the best bricklayer you have ever worked with. Everyone says that and they are not looking for the best, they want real, grounded skilled people with fact based responses not your opinions. Good luck.