According to Dow Jones, users have been drawn to 3 by its cheap calls, which 3 can offer because of its almost empty high-capacity network, but 3's clunky phones with their short battery life have not been a hit.
Users have been taking out the Sim cards and placing them in fashionable slimline handsets, which are then used to make ordinary voice calls. 3 says switching the Sim into another handset is barred under the terms and conditions of subscriber's contracts.
The problem for the Hutchison Whampoa-owned company is that a lot of the calls are not carried by it, but by the former BT mobile network O2.
O2 carries 3's calls in areas where 3G coverage is not working and 3 has to then pay O2 for every call its customers make.
The trend is also another blow for 3 UK in its battle to get users to take up its advanced 3G services. Users are telling the company they are clearly not interested in videophones. Last month, 3 revealed that it had signed up only 155,000 customers -- nowhere near its target of 1m by year-end.
A 3 spokesman told Dow Jones that it was monitoring users and if they were found to be using this technique, special software would be uploaded to their Sim card to prevent it working except in a 3 phone.
The spokesman told the news service that the number of subscribers taking advantage of the loophole was limited.
Internet message boards have been buzzing with news of the ploy, with many saying they do it because 3's 3G phones have poor performance.
One user quoted by Dow Jones said he was trying to use his 3 Sim in a Nokia 3310, a basic 2G phone, because "I only bought [3's 3G] phone so I could use the generous free minutes, but it's almost impossible to actually get through them, as it keeps dropping out."
Another user said: "Forcing people to use the 808 [one of 3's handsets] as a phone is the same as having a bath in a washing machine. Sure, it's got all the features but it's not suitable for the purpose."
In July, Hutchison was forced to demonstrate the picture quality of its video mobile phone handsets to the ITC after viewers complained that television ads showed much better quality images than in reality.
Viewers complained that an ad for 3, created by TBWA\London, featured clips from Premiership football matches that were much of higher quality than the service was able to achieve.
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