My husband earns an hourly wage for his 40hr/week supervisory job. He works in the grocery retail business. Almost daily, fellow co-workers and supervisors call him at home while he is off the clock to discuss incidents where people didn't show up, deliveries didn't show, etc. Sometimes these calls come late at night (after 9 or 10pm). They aren't calling to request that he come in, but rather to get direction, etc. It is stressful to have to deal with work issues while not being paid to deal with them. My question is whether it's legal for them to be calling him while off the clock.

It's legal because there is no law that prohibits it.

However, the question is whether he needs to be paid for that time. Has he brought this up to his manager? How often is he called and how much time is spent on the calls? What happens if he just doesn't answer the phone?

The specifics are probably less important than the culture. The company's attitude is such that if the job (however unreasonable) does not get done, then they'll find someone else to do it. This results in up to 10 hrs/week of unpaid work from home and goes undiscussed for the above reason that if it's not done in 40 hours, he'll lose his job.

However, the specifics can make the difference as to whether he should be paid for the time or not.

It is NOT illegal to call him at home. It MIGHT be illegal for the time to be unpaid.