Alright so I am doing this off of my guess that you did not do anything to mod your iphone or siri with a 3rd party application, If so then that might be the cause.
Anyways, are your contacts correct, is the phone number correct? Just double check. Secondly turn off everything that haas to do with location and connection to internet, turn off the following: wifi, anything with to do with your carriers service like 3g etc. Also try and turn off siri, you can do this by going into settings>general>siri>off. Now reboot your iphone, dont just slide to power down, but completely turn it off by holding the home button and the sleep/wake button until your iphone turns off. Then turn it back on. You may now turn on siri and whatever else you desire. Now that you have turned siri back on again, try it one more time.
Keep in mind Siri is in beta, and there will be some kinks and bugs to it, and is not gauranteed to work 100% of the time, like diesel vdub had said. Also it may just be a bug with siri, and maybe you should report it to apple. I will try and look up some more information for you too if i can.
But, if it is one of those thigns that you need to fix, and you are willing to do so, try and restore you iphone completely. This might help but nothign is for sure, but dont restore if you have valuable data that is not somewhere else like on your computer, or synced with icloud, before doing a restore. If you do chose to do a restore, make sure you do a backup first!! then just restore it completely.
Keep in mind that i am not 100% sure that what i have said will work, and i might be wrong. But i hope i helped. Im not saying that i dont have a clue as to what im talking about, but rather that i cant be sure until it happens.