Welsh phones pick up Irish network data charges
Mobile users on the north Wales coast have run-up extra charges after their phones connected to Irish networks.
One couple said it can cost £20 more a month when the problem happens at some trouble spots on Anglesey.
Experts warned people to check their phones to avoid racking up roaming costs.
Phone company Vodafone apologised to customers and advised them to change their settings to stop it happening again.
"We can be standing firmly on Welsh land, and yet we're being charged extortionate amounts of money," said Alan Davies, from Llandudno, Conwy county.
He and partner Ruth Miller regularly visit places such as South Stack, near Holyhead at Anglesey.
He told BBC Radio Wales' Jason Mohammad programme his phone picks up an Irish mobile network once or twice a month, but warning text messages do not always come through until later.
A Vodafone spokesman said: "Unfortunately, radio waves do not recognise national borders and we do occasionally see incidents of this nature."
Ann Durrell, from Manchester, was on holiday on Anglesey when she was charged extra.
She said: "I didn't receive any warning that my phone had locked on to an Irish network. I just happened to check my bill when I got home."
Telecoms expert Ernest Doku from uSwitch.com said the problem also happens on the border between Northern Ireland and the Irish Republic, and on the south coast of England.
"It happens to all phone networks. Sometimes the local geography can block the UK phone signal," he said.