I would first ask the person why they believe this way.
Perhaps I had said something in the past that they
misinterpreted, or perhaps they simply do not yet
know me well.
After they explained why they do not find me credible, I
would describe my qualifications in whatever
area was necessary.
If this still did not work, and my company was in danger
of losing a client or customer, I would turn the
customer over to a co-worker or manager.
It is not as important for ME to maintain the customer
as it is for US to maintain them.