Hidden mobile charges that could be buried in your bill
When Maria Dementieva, a 56-year-old doctor in Moscow, dials her husband's mobile phone number, she hears music instead of a normal ringtone.
It's the same for him. What they don't know is that this "service" costs them money, even though neither of them recalls subscribing to it.
Only when Mrs Dementieva calls her mobile phone network MTS, Russia's largest mobile operator, does she find out that she is being charged for it: 42 roubles (80p; $1.30) a month.
When she complains that she never bought the service, the customer service representative calmly replies that yes, she did.
She is experiencing the same issues as phone users in many other countries.
"Turns out that shortly after I had signed a contract with them, they sent me an SMS, informing me that I had this music added to my account and that it was free for the first month, but after that it was going to continue automatically with a monthly fee," Mrs Dementieva says.
"And to stop it, I had to call them and cancel it.
"But I only use my mobile for calls. It is not a smartphone, the screen is tiny and I am not tech-savvy, so I don't read or send SMS.
"I guess I had received this text but I never read it. And I certainly did not know I was being charged for it."
Two of Russia's other big telecom operators, Megafon and Beeline, have similar services.
Such practices have enraged users, and Russian online forums are peppered with angry comments from customers claiming that the services they never chose to have are costing them dearly.
One blogger, Roman, writes that he bought an MTS Sim card to use for web access only - and still got charged for phone ringtones.
MTS declined to comment.
In the UK, industry watchdog Phonepayplus states that users "must not be charged for premium rate services without their consent" - and evidence of consent is required.
"Where consumers are accessing high-cost subscription services, we have a system of double opt-in to make certain that they understand the full cost of the service," says Patrick Guthrie, director of strategy and communications at PhonepayPlus.
To avoid hidden charges, Mr Guthrie says people should always read the terms and conditions in any advert.
"Once you've seen how a service works and what it will cost, then make up your mind about taking part," he says. "Be wary of 'free' offers or fabulous prizes. If something looks too good to be true, it probably will be.
"If you're signed up to a subscription service on your mobile but want to quit, simply send the word STOP to the service number."
The organisation says that teenagers are especially likely to click on a service that promises to be free, such as a trendy ringtone, but do not read terms and conditions - until their parents discover a hefty charge on the monthly bill.
PhonepayPlus has even set up a website called PhoneBrain to educate young people about the potential pitfalls of owning a mobile.
When people move from a normal mobile phone to a smartphone, many may not realise that there are a lot of extra hidden fees, says analyst Carolina Milanesi from research firm Gartner.
"People should be aware that smartphone apps that are left active keep pushing content to the [phone], which may result in extra charges," she says.
"I am not sure consumers necessarily realise that and this is especially true for users who suddenly have to deal with a device that is more clever than they thought.
"This can easily be avoided by making clear that the app is using the mobile network, something that most apps already do.
"Other things like backing up photos or files can also be set up, as well as working over wi-fi in order to avoid charges. Maps used when roaming are another source of cost when those maps cannot be downloaded beforehand."
Cramming and scamming
In the United States, millions of customers find unauthorised fees from third-party firms on their phone bills, according to the Federal Trade Commission (FTC), a government agency.
This is known as "cramming," because these phony charges are "crammed" among many items on a monthly bill and may be hard to spot.
Customers end up paying for services they have never signed up for, ranging from ringtones, horoscopes and music downloads to yoga classes and diet plans.
Landline cramming affects 15 to 20 million US households, says the FTC - and now mobile phone scams are becoming a real issue too.
According to a US Senate inquiry completed in 2011, cramming costs American consumers about $2bn a year, and only one in 20 realises they have been scammed.
The report states that leading US telecom companies Verizon, AT&T and CenturyLink/Quest are not only aware of the issue, but pocket millions from allowing cramming charges on their bills.
Although there is no legislation yet to deal with cramming, the FTC recommends "that wireless providers be required to offer consumers the ability to block third-party charges, and to make this option clear to consumer".
Both AT&T and Verizon have recently promised to limit the ability of third-party firms to put charges on their bills.
Back home in Moscow, Mrs Dementieva is happy she no longer has to pay for something she never chose to have.
And in future, she says, she'll keep a closer eye on her monthly bill and on incoming texts.
Even if she has to read them on her handset's tiny screen.