Depends on the airline and the location as to if they use ACARS or VDLM2. (On a related note for anyone who monitors them, I just submitted the new channel plan to the RadioReference nationwide database). I do hear a lot of voice traffic with the explanation of "our ACARS isn't working."
Good point. To clarify upon what I said in post #4: most of the planes at my carrier have VDL2 installed, and a handful of our oldest planes still have legacy ACARS only. All of the airplanes with VDL2 installed will use that as their primary method of datalink transmission but still have legacy ACARS available for transmission.
As the end user, the differences between the two modes of datalink transmission are completely transparent, except for the fact the datalink page will momentarily display "NO COMM" as it transitions from one mode to another. The jump from one mode to another is completely automatic (likewise the frequency switching that is required is also automatic).
But with all that said, Delta's Atlanta Radio or ARINC's San Francisco radio is always available when needed (or more convenient than texting).
Edit to add: these radio networks are required for the airlines by federal regulation
14 CFR 121.99(a). Atlanta Radio and San Francisco Radio are both ways to comply with this rule.