Smartphones do not emit Co2 themselves, but they do cause Co2 emissions.
1) They require energy to charge the phone. Energy (often) means fossils fuel are burned.
2) The production and shipping of your phone requires energy or transportation that requires burning fuel.
3) The infrastructure supporting your phone (phone masts, network routers, etc) also needs energy.
We are considering offering this as an option, but at this moment we aim to limit the Co2 reduction to the energy used by the phone only. If you are interested, you can find an interesting article on Apple Iphones here.
We are also considering this as an option, but need to look into this in more detail. At the moment we first aim to reduce the Co2 emissions caused by charging a smartphone. If you are interested in this topic, you can read more here.
Other sources on Co2 emissions