ATC do not get any indications that you are "on frequency" so voice comms must be established first. You will be given voice instructions more often than not as many instructions require immediate action and cannot suffer the 40 second delay possible if using CPDLC. CPDLC has some uses that can save valuable RTF on certain frequencies, such as obtaining requested flight level, issuing routings and also transferring to the next frequency, all can be done simply whilst doing other tasks with other aircraft.
I have had aircraft tell me they are "logged on, CPDLC available" and when the message sent failed, worked out they are logged onto a different centre who they are about to contact
It's never as simple as it may seem.