She admitted she had hung up on customers before but was not the only employee to have done so.
The other two workers apologised, and were disciplined.
But the company made further enquiries about Ms Zhang, pressing her with more complaints.
These related misconduct claims were laid out in a letter given to her on May 27.
Ms Zhang later emailed her employers, apologised for all the issues raised, and pledged to be more careful in future.
But soon Ms Zhang started having stomach pains. On a midwife's advice, she was told to take a week's bed rest.
She returned to work on June 10 and was told to attend another meeting.
Her bosses questioned her sincerity, accusing her of apologising for lengthy meal breaks too late.
She was told she might be suspended on pay while the firm decided what to do.
But Ms Zhang left work around mid-afternoon that day, experiencing bleeding. She went to hospital.
An ultrasound exam the next day confirmed the pregnancy had miscarried, possibly a few weeks earlier.
On June 27, Ms Zhang was handed a letter of dismissal.
Ms Zhang told the Authority her sacking was unjustified, the GL Futures investigation was flawed and the firm accused her of being dishonest.
She sought remedies of lost wages, distress compensation and a penalty for "numerous breaches of duties of good faith".
GL Futures said its investigation was thorough, with allegations 'squarely put' to Ms Zhang.
The firm said Ms Zhang admitted exceeding her meal break and jeopardised the company's reputation by "rudeness and ill-treatment" of customers.
The company also said the decision to sack Ms Zhang was made in Hong Kong, where the director and his assistant were based.
The Authority said Ms Zhang was treated unfairly, compared to the two other disciplined workers.
They also ruled Ms Zhang was unjustifiably dismissed and awarded lost wages of $9216.
GL Futures were ordered to pay to pay $9000 compensation for humiliation, loss of dignity, and hurt feelings.
Ms Zhang's lawyer Zhenzhen Chen said she'd seek instructions from her client before commenting.
GL Futures ' lawyer David Liu and company director Man Bok Mui could not immediately be reached for comment. GL Futures' lawyer David Liu said he was not yet sure if his client would appeal.
He said from a "professional perspective" he was a bit surprised the Authority felt 50-odd dropped calls were a "performance issue" rather than a disciplinary matter.
Mr Liu said some of these customers would have been waiting a long time so to be hung up on would be extremely frustrating.