Unfortunately Fergie's cell phone number is not on public record so I very much doubt anyone on here would know it and more importantly, would be willing to share it.
Can you imagine how many people would call a star if their number became public knowledge? The network would probably get blocked in a matter of minutes and poor Fergie wouldn't have time to do what she does best, getting on with her music career.
The other thing is that I would never give out anyone's number without first getting their permission, so if anyone did have it I am pretty sure they wouldn't be passing it on to people, it's a pity but that's the way it is.
However, if you know George Clooney's number, do let me know, I promise I won't call when his wife is around :-)