The BT Group has said it will do "whatever it takes to protect 999 services" during a strike by the Communication Workers Union (CWU) in Northern Ireland.
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Staff who answer emergency calls are among hundreds of BT Group staff in NI who began strike action on Thursday.
CWU said most of its members at BT's 999 call centre would strike.
About 120 call handlers are the first point of contact for the public in an emergency situation.
Those workers, based in County Armagh, then relay calls to the appropriate service such as the police, the fire service, or the ambulance service.
"We have about 1,600 people in BT/Openreach in Northern Ireland and we expect the majority of them to be on strike," CWU NI Regional Secretary Erin Massey said.
"We were on strike before, but as part of that agreement the 999 emergency handlers were not on strike.
"We have to show the company we are serious.
"It is not an easy decision for us to make, for the emergency call handlers to go on strike, but the company has given us no other choice.
"They are ignoring our demands to come to the table."
Ms Massey said workers "want a decent pay rise, not the pay rise of 2% imposed by the company" which she said represented "a pay cut when measured against the cost of living increases".
