An Alberta-based discount phone company with an "F" rating from the Better Business Bureau is again the subject of complaints by customers who say they were overcharged.

The latest allegations are that Calgary-based VOIS hiked its per-minute rates by at least 15 times the amount that some customers thought they were going to be charged.

Edmontonian Florence Juan said she was thrilled when she discovered a discount phone company with rates so cheap she could afford frequent calls to her family in South Sudan.

"The rate was really good," said Juan. "I signed up because the deal was really good."

Juan signed up with VOIS in October after hearing about the low rates from others in her community, something she confirmed by examining a friend's bill.

She said a customer service agent told her she would be charged three cents per minute. In fact, her November bill shows the rate was less than one cent, with long distance charges totalling $27.41.

But the following month, despite much less calling time to South Sudan, her long distance charges shot up to $917.09. The per-minute charge to South Sudan was often as high as 44 cents.

Juan said she should have been warned about the increase but never was.

'I feel like I'm being robbed'

"I feel like I'm being robbed," Juan told CBC News. "They're trying to give you a good deal, then after that, they change their mind without notifying their customers."

She said the bill is as much as she makes in two weeks as a community disability worker, leaving little left for her to pay bills and support her late brother's children and her younger siblings overseas.

Juan said when she called VOIS, an agent acknowledged a rate hike of 42 cents per minute and admitted customers had not been given notice.

"They told me they have a lot of customers, so it is difficult to mail notification to all their customers", she said.

The agent told her to pay half, offering to waive the remaining charges, said Juan. When she said that was still unaffordable, the agent hung up on her.

In a separate call, witnessed by CBC News, a VOIS customer service agent told Juan the per-minute rate had risen to 48 cents. When Juan asked what the rate was before the increase, he also hung up.

Juan is one of several VOIS customers in Alberta who told CBC they are on the hook for overseas charges they never agreed to.

They all said a VOIS agent told them rates to South Sudan were three cents or less, although their stories suggest they were given different rates, and often charged even less.

In Calgary, father of four Gabriel Bongily said he felt "deceived" when his bill suddenly increased from $22 to $117.

"I told them that it is not fair, because if there are changes they could let me know," said Bongily. "They should not let me continue using the system and then later send a bill that I did not expect."

He ended up paying his bill of $117 when VOIS cut off his service, he said.

'Mistake in the billing software'

On Monday, Harpreet Randhawa, director of VOIS, blamed the abrupt billing changes on a technical "billing mistake."

"Prices didn't go up," said Randhawa. "There was a mistake in the billing software."

He said VOIS had mistakenly undercharged over the past four months, using per-minute rates of far less than one cent.

"And finally we figured this out and we fixed our billing," said Randhawa.

When asked why customers were not warned about the increase, he said: "We never told these customers South Sudan is less than one cent."

Randhawa said rates range from 25 to 45 cents, depending on the individual deal. But because of the error, customers had basically been given "free long distance," he said.

"We are not sending their bills to collections but we can't continue with this, and most of these customers want one cent. That's not possible."

Randhawa said VOIS is "willing to work with every single customer" on the billing problem.

When asked about allegations of agents hanging up on customers, he said he didn't know why that happened because he "was not on the call" and had not been asked to review the exchange in advance.

In 2013, CBC News reported on an Edmonton father's eight-month battle with VOIS after he mistakenly overpaid $1,300.

VOIS agreed to reimburse Mazhar Malik after CBC called the company for an explanation.

Business bureau "F" rating

The Better Business Bureau gives VOIS an "F" rating for failing to respond to six complaints in 2013 and 2014 that include poor customer service, failing to refund customers, discrepancies in bills, and issues around cancelling the service.

"If there is any kind of discrepancy or change in a price, consumers should be made aware of that up front, so that they do have that information to make an informed decision," said bureau spokesperson Leah Brownridge.

Brownridge advised those with grievances to file complaints to the bureau.

She said they can also contact the independent watchdog agency called the Commissioner for Complaints with Telecommunications Services, which has "more clout to order companies to comply with certain standards and business practices."