For at least some Amish I think the main rule is they're not allowed to enter contracts. So they can buy gas and batteries but they can't have mains electricity, they can use a phone as long as it's pay-as-you-go. I'm pretty sure that varies between groups; each Amish community basically has its own rules. I know some of them don't use electricity at all, but they can use pneumatic power tools. And there's still a fair few pay phones in Amish country they can use to call doctors and veterinarians and people like that. There's also other groups of Anabaptists like Mennonites that are similar to Amish in some ways but different in others, even some that drive cars and work in offices but just always wear long sleeves no matter the weather.