Business phones were cut off and elderly people were left stranded without panic buttons as an attempted cable theft in Watford disconnected hundreds of lines.
BT engineers are working to restore phone and internet access following "malicious" cuts to underground cabling last week.
As well as affecting homes and businesses, the outage had an impact on alarms elderly carry to contact emergency services in case of an urgent need.
Tony Herman was left concerned about his 95-year-old mother-in law who wears a button around her neck which connects to the phone line when there is an emergency.
Her Kings Avenue phone lines are down and he said he is worried about her safety.
Mr Herman said: "She wears a life line around her neck and she has never used a mobile phone. All she has to do when there is an emergency is press the button round her neck which connects to BT and now that’s not working."
People across the town lost internet and phone access after criminals used bolt cutters to slash copper cables under a manhole cover in Whippendell Road.
The crime, which took place between Addiscombe Road and Park Avenue, was reported to Hertfordshire Constabulary at about 2pm last Friday.
BT has received around 500 reports of damage, in which some have called a "nightmare" situation.
Yasmin Shahrad, a teacher and Whippendell Road resident, said: "It has been pretty awful. For nearly a whole week I’ve had to go and sit in cafes and go round friends houses in the evening and at the weekend just to do some work.
"It has been a nightmare and it’s making life really difficult."
Businesses have also been affected. Kevin Martindale, managing director of JPM Contract Cleaners, in Rickmansworth Road, said the office’s lines went down at lunchtime last week.
Mr Martindale said: "The whole office lost the phone system and the internet.
"We have got 1,400 staff as well as all of our clients across the UK and no-one can get through. We have had to forward the calls through to one single mobile.
"We’re just sitting, looking at each other. We might have to bring some board games in."
Paul Hayward, spokesman for BT, said: "We have suffered malicious cuts to underground cabling, in an apparent attempted theft of cables, which has been affecting some services in the area at the moment.
"Engineers have been onsite working to repair the damage and changing out the damaged sections of cabling.
"We have had around 490 reported faults from customers that were affected by this attack on the network. Some people have now had their services restored and this will increase and reconnection works progress. We are doing all we can to get this work completed as quickly as possible."