How often is too often?

When being asked by a real estate agent to contact one of their clients, how often is too often to call them back.  I was asked by a REA that I work with to contact a client of his regarding getting their home staged, he said they were expecting my call and needed my services.  I called the number he gave me and got an answering machine (with just the original computer recording voice on it).  Do I call back after a few days?  They have yet to return my call (4 days later).  Do I call the real estate agent?  Do I double check that the number is right?  I don't want to seem pushy and in their face, but if the number was wrong or if they didn't get the message I also don't want to seem uninterested/unreliable.  I'm in a pickle here.