A 45ft mast put up in Bucks without planning consent has been allowed to remain because of a legal loophole.
O2 found its planning consent was refused a day after the deadline
Wycombe District Council's refusal of O2's plan was backed at an appeal hearing, but the company put up the mast in High Wycombe late last month.
O2 established that because the council was a day over the 56-day deadline in responding to its application, the firm had "permitted development rights".
Residents of Amersham Road and Arnison Avenue said the mast is an eyesore.
Stephen Chaffin, whose home is about 20 metres from the mast, said: "I moved to the area just over a year ago and so was shocked to wake up to a 45ft (13.7m) telephone mast having been put up directly across the road from my house - so were my neighbours."
He said he had taken up the matter with his local councillors and MP Paul Goodman.
His neighbour Barry Evans, who has lived in his house for 30 years, said; "We just want to get rid of this eyesore and will take it as far as the local government ombudsman.
"Having had the district council refuse planning consent in 2003 and an appeal in 2004, how can O2 think it is OK in 2005?"
In a statement the district council said its officers were aware that a mast had been erected and it was investigating the situation.
A letter from its legal services department, written in July 2005, to solicitors acting on behalf of O2 said the application was received by the council on 24 November 2003 but was not registered until 25 November 2003 "due to a couple of matters requiring further clarification".
The letter concedes that as the council decision was made a day after the 56-day deadline, O2 had permitted development rights to erect the mast.
O2 regional communications manager James Stevenson, said: "Somewhere along the line our solicitors ascertained that they (Wycombe District Council) had got it wrong."
He maintained that work would be completed on the mast which was necessary to serve O2's customers.