The action has been called after BT and Openreach workers refused a £1,500 pay offer.
The Communication Workers Union (CWU) said the action will be the first national telecoms strike since 1987.
It has warned the strikes could affect the rollout of ultra-fast broadband, and cause issues for people working from home.
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But Ernest Doku, broadband expert at Uswitch.com, has useful advice for broadband customers who are worried.
He said: “BT has a third of the UK’s fixed broadband subscribers, and if you’re an existing customer experiencing an internet outage or looking to have BT broadband installed, it’s possible that the strikes could affect the service you receive.
“Contact centre staff will be taking industrial action today and Monday, so if you have trouble with your connection you should visit the service status page on the BT website, which should show if there are network problems or the issue is specific to your line.
“The provider’s Twitter page can also be a useful source of service information, while a site like DownDetector can indicate if other customers are reporting similar faults.
“Service disruption can often start at home and it’s always worth checking and resetting your router first, just in case an outage is linked to your equipment.
“As engineers for the Openreach network will also be on strike, households hoping to have BT broadband set up this weekend should be prepared for rescheduled dates or cancellations, although this should not impact consumers who get their broadband through BT-owned EE.”
More than 40,000 workers are expected to walk out with 260 picket lines set up across the country today, with a second strike planned for Monday.
The strike action comes a day after BT reported that its revenues increased by 1% to £5.1bn for the three months to the end of June.
A BT spokesman said that while bosses respected the choice of CWU members to strike, the company would work to minimise any disruption and keep customers and the country connected.