Setting 500 to go to Voxalot in the dial plan
 

I set up a line in the dial plan towards the end of the list to send "500" to Voxalot.com. When I test it on the dial plan page, I get the following message :

[A call to:500 will get forwarded to sip:500@voxalot.com]

However, it does not work when I try it from the phone (GS Budge Tone 102).
I get a 603 code on the screen and a busy tone.

If I remove it, I can access my VM by dialling 500.

Although this is not really a problem, I am curious as to why the dial plan behaves like that.

Can anybody shed some light on this?

Thanks