Across the UK, 40,000 Communications Workers Union (CWU) engineers and call centre workers within the company have walked out.
The CWU said it is the first national telecoms strike since 1987 and the biggest ever among call centre workers.
A further strike will be held on Monday after union members voted in favour of industrial action in protest at a £1,500 pay rise.
On Friday picket lines will be in place at sites across the country including Alexander Bain house in Glasgow.
Speaking to BBC’s Good Morning Scotland on Friday, the CWU’s Scottish regional secretary, Craig Anderson, said: “Our members in BT and Openreach are taking industrial action on Friday and Monday over pay.
“We went into negotiations with the company and they offered a very small pay rise. The union felt it wasn’t enough under the current cost-of-living crisis.
“There was no consultation, no negotiation and no vote or acceptance from members and the company has just imposed what they think is a reasonable amount of money for our workers.
“We’re willing at any point to get back round the negotiating table to discuss, to negotiate and to actually settle this dispute.
“The last thing our members want to do is go on strike. The last thing our members want to do is inconvenience members of the public and the last thing our members want to do is lose a day’s wages, but there is no choice.”
Mr Anderson warned that customers expecting work to be done on their broadband or phone lines on Friday and Monday could face disruption.
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 said it was also boosted by more people signing up for fibre-optic broadband and 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.
Dave Ward, CWU general secretary, accused BT of “gaslighting” members.
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.
“This dispute sits squarely at the feet of chief executive Philip Jansen. He represents everything that needs to change about big business in Britain.
“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.”
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.”