If it is for business it is generally allowed by the ordnung, which is set by their local bishop. But sometimes that phone can't be in their homes, it has to be in a separate building, or in their place of business.
"Because the Amish have no central church government, each assembly is autonomous and is its own governing authority."
Because of this a lot of variety exists.
I have seen pneumatic and hydraulic systems allowed where electricity is not allowed. If the air used happens to be compressed using electricity by a non Amish American neighbor, it can be allowed. It can seem very arbitrary to an outside observer...
I am surprised how many Amish and Mennonites seem to enjoy and participate in Steam Traction Engine Shows in Ohio. I would have thought their ancestors would have looked negatively upon steam machinery in the 1800s.
-CB
P.S. I will say that the Amish and Mennonites always seem to be extremely dedicated strong laborers who are honest and trust worthy. In Chautauqua my family several time payed for an Amish man to paint our house in Point Chautauqua. Also, our house there was made in 1879 when 2" x 4"s were really two inches by four inches. To match the framing while renovating the house my dad had the nearby Amish lumber mill cut us proper 2" x 4"s. They then required a year? to dry in our back yard - but in the end it matched the original framing.