I am a pretty heavy business traveler and stay in moderate priced motels to 4 star hotels and I get follow up calls from the front desk from many of them. Don't know why your client got spooked, maybe he was a newbie. Next time you check in, you might tell the front desk no need to call the room.

mid to upper scale hotels; especially if you are a member of their frequent guest program.  

If I'm traveling on business and are having company in my room and don't want to be disturbed, I pull the phone cord out of the back of the phones.

Take the phone off the hook.

That's how I cured it.

Yes, it happens in many hotels.  You can avoid this by calling the hotel operator when you get to your room and telling him/her that you do not want to be disturbed.

Sounds like your guest was a bit paranoid.

then he would know this is SOP at the nicer hotels.  Where does he usually stay, Super 8?

I've also had them while traveling with my family.  I didn't thik anything of it at the time but can see how it would be disturbing.  There's only 1 phone in most rooms...  I agree unplug it.  
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Tom Bodet(sp?) must be leaving a light on for that guy if he didn't know that a courtesy call from the front desk is in fact quite common.

Pretty much standard practice at 4 **** Hotels worldwide. I kind of like it. Just mute the phone.

Too bad that he got spooked.  I get one everytime I check in too....definitely take the phone off the hook or just tell them not to "courtesy call" you.
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yes many hotels do this. Just tell them all is fine end of story. Hampton Hilton and Sheraton are famous for that.