The Communication Workers Union (CWU), which represents BT workers, are on strike today (October 6).
Emergency 999 call operators were initially exempt from the strike action, but changed their minds after "widespread outrage at the company’s refusal to negotiate".
Around 500 emergency call handlers will walk out for the first time as they join their colleagues - 30,000 Openreach engineers and 10,000 BT call centre workers.
Further walkouts are currently planned to take place on 10, 20 and 24 October amid warnings of what could be a "dangerous dispute for many".
Here’s everything you need to know and how the strikes may impact you.
Why are emergency 999 call operators striking?
Action has been taken in opposition to BT Group management’s offer of a flat-rate pay rise of £1,500.
BT said that this represents a pay rise of approximately 5% on average, and 8% for the lowest paid workers.
But, in the context of RPI inflation levels hitting 11.7% this year, the CWU says the offer is a "dramatic real-terms pay cut".
Dave Ward, CWU general secretary, said: “This decision was not taken lightly, but our union’s repeated attempts to initiate discussions was declined by a management who clearly believe they are above negotiating a fair deal for people who make massive profits for them.
"999 operators are using food banks, they’re worried about the cost of living and are being stretched to the limit.
“BT management clearly has no appreciation of its workforce, or their social value, but this union does, and we will keep on fighting this company’s attempt to plunge its workers into even further hardship.”
CWU deputy general secretary Andy Kerr said: "Time and time again we have asked for negotiations to resolve what may well be a dangerous dispute for many, only to be ignored by a senior management that could not be less interested in addressing the anger that exists in their workplaces."
Who is replacing emergency 999 call operators?
BT told Sky News it has "up to 150 call handlers on shift at any one time in our 999 contact centres and we would look to maintain those levels by redeploying colleagues".
The company added it will have "experienced agency teams" working on strike days and "expect a number of our colleagues" will also attend work as normal.
It has also "been training colleagues" to "fill gaps from any absences".
How do 999 emergency call operators link to BT?
Six BT call centres handle all the UK’s 999 calls in Northern Ireland, Scotland, Wales and England, passing calls to the relevant emergency service.
Have BT commented on the strikes?
Yes - a BT Group spokesperson said: "We know that our colleagues are dealing with the impacts of high inflation and, whilst we respect the right of colleagues to take industrial action, we are profoundly disappointed that the CWU is prepared to take this reckless course of action by including 999 services in strikes.
"We will do whatever it takes to protect 999 services - redeploying our people to the most important priority is a normal part of BT Group operations.
"We made the best pay award we could in April and we have held discussions with the CWU to find a way forward from here.
"In the meantime, we will continue to work to minimise any disruption and keep our customers and the country connected."