Navient/SallieMae Harrassment Internet

I am a cosigner of a student loan with my grandson with SallieMae/Navient.  I get empty voicemail messages on my home phone from them - sometimes 3 times a day attempting to reach me regarding payments being late less than two weeks. Everytime I get an empty voicemail message from them I contact my daughter and she tells me that the payment was either made or would be made within the next two days.   My grandson who is the primary loan holder told me that they don't even contact him.  They have now begun to call my cell phone.  When they called my cell, they asked if it was ok to use the cell number and I told them no.  I blocked the number on my cell and they called my cell again with a different number.  This is total harrassment.  They should be contacting my grandson first and if they can't contact him, then they should try the cosigner and leave a message.