Some cell phone service providers are warning their customers not to provide phone numbers to text-based subscription services such as jokes of the day or IQ tests, because they can add up to significant extra charges.

Student Amy Cisyk learned the hard way.

She clicked on an ad that popped up while she was checking her social networking account.

"I was doing another quiz and it said 'your friend took this quiz, and she got this score - see if you can beat it.' And so I clicked on it and started doing the test," she told CTV News.

She answered about six questions, and at the end it asked her for her cell phone number, promising the results would be sent via text message.

She entered her number without realizing that if she had scrolled down she would have seen the fine print that stated she would be charged for signing up.

The next day Cisyk got the results of the IQ test in a text message.

Then everyday after that an IQ question was texted to her cell phone.

Thinking she would get charged, she never responded to them, but Cisyk found out later the messages would cost her anyway.

"I got my bill and noticed they were charging me $2 for every text they sent me," she said.

She got 19 texts altogether, costing her almost $40.

Service provider Telus says there are two kinds of text messages, those sent between customers and those generated by subscriptions for a service.

IQ tests, jokes of the day, or even news updates are called short code service premiums.

Even if you have a text messaging package in your plan, these messages are different and will cost you money.

The cell phone provider acts only as a billing agent for those short code services, and they can not waive the fees.

The good news is there is an easy way to end the subscription, just text the word "stop," to the sender.

That is what Cisyk did and the messages stopped. It was a frustrating lesson to learn for someone just trying to test their brain power.

"I obviously didn't have a high enough score to figure this one out," she says.

Security analysts recommend consumers not give out any personal info over the internet to any company or source they do not already know or trust.

They say also recommend looking for more the "Terms and Conditions," section on websites.

They say to be very cautious if there are not terms and conditions, because that means the website is not giving an explanation of what they may do with the information.