Two big cranes are at the Warbreck water tower, with workers set to spend around a week there.
They are upgrading the mobile phone mast equipment attached to the side of the structure for the O2 and Vodafone networks, a spokesman for owner United Utilities confirmed.
The cranes were at the tower, in Leys Road, yesterday to 'enable the telecommunications equipment fixed to the side to be removed,' a spokesman for the water firm said.
He said the work will take around one week.
In a statement to The Gazette, O2 said: "Vodafone and O2 customers expect to be able to use their mobiles and devices where they live, work and travel.
"Base stations are low powered devices which cover approximately half a mile in radius, therefore we have to put base stations close to our customers.
"Vodafone and O2 identified that they need to improve coverage to their customers in North Shore, and are upgrading the existing base station on [the] Warbreck water tower.
"The upgraded base station will provide 2G, 3G and 4G services for Vodafone and O2 customers in the local area."
The largest crane, which is blue, is understood to have recently been working on South Pier, where a new 'Spinning Wild Mouse' rollercoaster has been built.
The water tower was built in 1931 and serves homes around the area by creating and maintaining water pressure.
It is fed with water from the Barnacre Reservoir in Longridge.