Calling frequencies are generally accepted ham agreements or Arrl (In my case) recommendations to where to make a general call for the various modes and activities. If after tuning the range for the mode you are using, no activity is heard, the calling frequencies are a good place to call CQ. After a contact is made it is best to move off of the calling frequency to continue the QSO so that others can use it for its intended purpose.
Whenever groups tend to congregate at particular frequencies, those frequencies are known as calling frequencies.
I did this just to see what other was thinking, I have been into QRP for over 35 yrs, started with a HW-8 and B&W window mount antenna.
REF: ARRl/Ham Radio for Dummies/AC6V's HF Ref Guide