Cannot Call Out to Specific Phone Number

I am not able to call out to a specific phone number from the iPhone.

I am able to call this specific phone number from different sources. ie. Another cell phone, lan line, work, etc...

Any clues on why just a specific phone number cannot be made to?

When I try to call, I always get 2 beep sounds only and says the Call Ended.
I get no Dial tone when I call also.

ps.
It could be that somehow they blocked my cell phone number.
But, I do not believe so.

-JK