You may be in a giving mood during the holidays, but donating to what you think is a charity could start draining your bank account a few dollars at a time.

It's called cramming, and it's a trick scammers use to add third-party charges to your phone bill.

"Many years ago, consumers didn't have credit cards or online banking, and the technology of your landline phone bill and now our cell phone bills allowed us to charge something to our telephone bill," Kathleen Calligan, from The Better Business Bureau, said. "Today, many of us have forgotten that, but if we look on our telephone bill, and I know we don't, you'll see that there can be third-party billing."

The charges are often for subscriptions to horoscope readings, love and dating advice or ringtones. If the consumer sees the charge and reports it to their phone company, the scammers can show that you've subscribed by playing for them a call where you said 'Yes".

"All you have to do is say 'yes' to something, just in general conversation. A scammer can pick that out of a recording and they can put third-party charges on your phone bill," Calligan explained.

How do the scammers get your 'yes'?

Earlier this year there was an increase in robocalls where the person on the other end of the line asked: "can you hear me?" The answer "yes" was recorded. Those 'can you hear me?' calls were being reported across the country.

The scammers are sly by adding the charges in $3 to $9 increments.

"So small every month and you won't even slightly notice a change in the bill, and it will stay there for years," Calligan said.

So how can you protect yourself?

If you get a robocall or a call with someone on the line asking anything that would draw a positive response such as "yes" or "okay", don't say anything and hang up the phone.

The FTC also urges consumers to report the call by going to www.ftc.gov.