Microsoft has released a new Insider Preview build of Windows 10 Mobile today, build 14283 which features improvements to the Phone app! This build has just started rolling out now to insiders on the fast ring, so if you can't see it just yet give it a bit of time. Today's release features mostly fixes, not many new features to take note of here.
This build also includes a new option in Cortana that allows her to sent phone notifications to a PC, but it is unknown if it works just yet. Like recent builds, this one is only available for the Lumia 950, 950XL, 550 and 650 as well as other Windows 10 handsets that shipped with W10M.
Microsoft is to detail a number of new features and developer oriented changes coming to Windows 10 at its BUILD developer conference later this month. The first wave of Redstone updates are set to rollout officially in June, with a second wave with more features coming a year later in Spring 2017. So far, RS1 includes improvements to Edge and built in messaging apps, as well as improvements to the OneCore subsystem.