Had this occur to me the other day. We got our PDC via the ACARS. At the end of the message it said "Acknowledge PDC" which was a selectable option on the FMC screen. It didn't make any mention of calling Clearance or giving a frequencey to read it back on. The Jepps do mention calling clearance and the 5 min call requirement. I recommend an addition be put on the bottom of the ACARS PDC directing one to call the appropriate freq. Oops, should have read what Slammer wrote. It was my SNAFU calling the 122 vice 121 ground freq.