Warnings over text-sharing apps
Warnings are being issued about Android apps that exploit a phone's ability to send unlimited numbers of text messages.
The apps are proving popular because they claim to reward phone owners for every text message they relay.
But, say security firms, the apps can relay huge numbers of texts via a phone and lead to its number being blocked.
Mobile operators say the apps violate their terms and conditions and could lead to a service being suspended.
The apps aim to profit via the unlimited texting that many people get with their mobile contract.
Routing text messages through a phone can be a cheap way to ensure an SMS reaches its destination, said Cahal McDaid from mobile security firm Adaptive Mobile.
"There's a huge market in sending text messages as cheaply as possible around the world," he said.
The fees for transporting text messages vary from country to country which has led to a "grey market" for routing messages via the cheapest route.
Routing the message via a phone already in the country that someone wants to reach could cut costs even more, he added.
However, said Mr McDaid, the sheer number of texts that some of the apps sent through a participating phone could cause problems.
Adaptive had seen thousands of messages passing through phones that had downloaded one of the apps.
In addition, he said, operators were likely to take a dim view of customers who use their phone as a text message relay.
"You cannot resell your message plan," he said. "Operators have terms and conditions for a reason."
A spokesperson for Bazuc, one of the message-relaying Android apps, said it told participants to ensure that the daily limit of messages they send is not set too high.
"We are fully aware that mobile operators are not going to be a big fan of this app," said the spokesperson. "We're simply trying to help people out there make some extra money."
The spokesperson claimed that some users of its app had five phones dedicated to using the app so they can cash in. Bazuc said it paid participants $0.001 cents for every message they relayed.
All the UK's large mobile operators contacted by the BBC said any customer using text-message-relaying apps would be breaching the terms of their contract. This could lead to their number being blocked or their service being suspended.
Marc Rogers, principal security researcher at mobile security firm Lookout, said getting caught using such apps could make the whole experience very expensive.
"If your operator decides to bill you their 'out of bundle' or overage rate for violating their terms you could be billed hundreds of pounds for those messages," he said.
"Aside from the potential issues with your operator, you are allowing people to send messages from your mobile number, without having any control or visibility of what those messages are," he said. "But you may have to face the consequences."