Phone-sitting

I've been "phone sitting" for a friend on holidays for the past week, and it has led to some interesting calls. The most recent one was particularly interest-inducing, it was the gas company, and they left a message on his voice mail while I was in class.

You have one new message. First message, sent today at 6:27pm from phone number 000-000-0000:

I almost hung up before she finished reading out all the zeros one by one. But I thought, maybe he was calling collect or something and I should probably listen. No such luck.

Hello, this is the gas company. We have not received your payment for this month's invoice. This must be resolved today. Thank you.

I kid you not. No name, no phone number, no outstanding balance, nothing. I have no way of responding to this. For past calls (and yes, it's happened), I explain that he is away on holidays and that I will be taking care of it. Usually no problem. But how do I respond to this? I drove to his house, looked for a bill, found nothing (for the record, we're close enough that that would be acceptable and that I would have a reasonable idea of where the bill is kept - I'm not some sort of snoopy stalker). I can't call. They didn't technically say they were going to disconnect the service. It was somewhat of an idle threat. If I didn't resolve it yesterday, they could do anything today. I'm sure I'll be getting a call from his friends staying there if they suddenly have no heat, though.

Zero. zero. zero. zero. zero. zero. zero. zero. zero. zero.