This page describes the plans to develop Plasma Mobile from a high-level. Day-to-day development and planning is done in KDE's Phabricator system.
The Plasma Mobile prototype shows the viability of Plasma on a handheld device. The prototype boots a reference device, is able to make phone calls, select contacts from an address book and contains partly functional outlook on how handset running Plasma could look like.
KDE Community Wiki 