The Communication Workers Union (CWU) said it marks the first national telecoms strike since 1987 and the biggest ever among call centre workers.
Another strike will be held on Monday (1 August) after union members voted in favour of industrial action in protest at a £1,500 pay rise.
The industrial action comes as BT revealed its first sales growth for five years on Thursday, as the telecoms giant benefited from price increases for customers earlier this year.
The group added that its profits were also boosted by more people signing up for fibre-optic broadband, along with strong trading in its Openreach network business.
BT told shareholders that revenues increased by 1% to £5.1 billion for the three months to June 30.
What has the union said?
CWU general secretary Dave Ward accused the BT Group of “gaslighting” its members by announcing profits on the eve of the mass strike action, and called for bosses to give workers a “proper pay rise” that they “deserve”.
He said: “Announcing hundreds of millions of pounds in profit on the eve of the first national strike since 1987 smacks of arrogance and complete contempt for frontline workers.
“BT workers have hundreds of picket lines arranged across the UK [today], and will support the CWU in delivering mass strike action.
“This dispute sits squarely at the feet of chief executive Philip Jansen. He represents everything that needs to change about big business in Britain.
“Our members kept the country connected during the pandemic. They deserve a proper pay rise, and that’s what they’re going to get.
“This is one of the worst examples of the corporate greed that people are so fed up with in Britain today.
“Workers at BT Group will never accept their bosses using Swiss banks while they use food banks. They deserve a proper pay rise, like all workers do.”
Has BT responded?
The group said it has been “trying hard” to reach an agreement with the CWU on pay and has “already made the best award” that it could, adding that the pay review for this year will not be reopened.
A BT Group spokesperson said: “At the start of this year, we were in exhaustive discussions with the CWU that lasted for two months, trying hard to reach an agreement on pay.
“When it became clear that we were not going to reach an accord, we took the decision to go ahead with awarding our team member and frontline colleagues the highest pay award in more than 20 years, effective April 1.
“We have confirmed to the CWU that we won’t be reopening the 2022 pay review, having already made the best award we could.
“We’re balancing the complex and competing demands of our stakeholders and that includes making once-in-a-generation investments to upgrade the country’s broadband and mobile networks, vital for the UK economy and for BT Group’s future – including our people.
“While we respect the choice of our colleagues who are CWU members to strike, we will work to minimise any disruption and keep our customers and the country connected.
“We have tried and tested processes for large-scale colleague absences to minimise any disruption for our customers and these were proved during the pandemic.”