A team of researchers from Washington University's computer science faculty have designed the device, which can be charged through tiny solar panels or from radio waves.
It also needs much less battery than a regular device as the team got rid of the element of a phone that takes the most power, converting a person's voice into digital data so it can be transmitted.
Professor Shyam Gollakota said: "We've built what we believe is the first functioning cellphone that consumes almost zero power. To achieve the really, really low power consumption that you need to run a phone by harvesting energy from the environment, we had to fundamentally rethink the design."
Whilst co-author Professor Joshua Smith added: "The cellphone is the device we depend on most today. So if there were one device you'd want to be able to use without batteries, it is the cellphone. The proof of concept we've developed is exciting today, and we think it could impact everyday devices in the future."
In an abstract for their paper, published in the Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies journal, they write: "We present the first battery-free cellphone design that consumes only a few micro-watts of power. Our design can sense speech, actuate the earphones, and switch between uplink and downlink communications, all in real time. Our system optimizes transmission and reception of speech while simultaneously harvesting power which enables the battery-free cellphone to operate continuously.
"The battery-free device prototype is built using commercial-off-the-shelf components on a printed circuit board. It can operate on power that is harvested from RF signals transmitted by a basestation 31 feet (9.4 m) away. Further, using power harvested from ambient light with tiny photodiodes, we show that our device can communicate with a basestation that is 50 feet (15.2 m) away.
"Finally, we perform the first Skype call using a battery-free phone over a cellular network, via our custom bridged basestation. This we believe is a major leap in the capability of battery-free devices and a step towards a fully functional battery-free cellphone."