If I'm being called back to work after a layoff, are they supposed to hire me back through the company or a temp agency?

Once you are laid off the company has no obligation to hire you.  if the company does in fact decide that they want you back, they can call you directly and hire you.  if they have an agreement with the temp agency where people that are laid off and return to work must go through the temp services, then the employer or temp service may contact you.  Also, i would need to know the specific terms if any that you took a voluntary layoff.  For instance, it there a contract or something in writing relating to the voluntary lawoff?  did you receive any thing in exhange for taking the layoff?  I would just sit tight and wait to hear from someone.  In the meantime, you should look for other employment.