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I use the old phone ring tone built into the phone. Not only is it loud it also doesn't sound like a college kids ringtone.
 
Haha, business man got banned! :p Those 4 sentences (he cross-posted this) did so much damage. ;)

As far as ringtones go... The most professional that I know of is Vibrate. Although the iPhone does tend to get switched off vibrate sometimes. I wish there was a confirm dialog when this happens to make sure it's not on accident.
 
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