Hello,  
Yesterday we released Adaxes 2011.2 that allows you to validate user input using scripts.
To validate that the specified telephone number is unique in the directory, you need to create a Business Rule that will be triggered before creation of new users in AD. This Business Rule should execute the following PowerShell script:
Import-Module Adaxes
if ($Context.IsPropertyModified("telephoneNumber"))
{
    # Get the value specified by the user
    $value = $Context.GetModifiedPropertyValue("telephoneNumber");
    # Ensure that the specified phone number is unique
    if ((Get-AdmUser -Filter 'telephoneNumber -eq $value') -ne $NULL)
    {
        $Context.Cancel("A user with the specified telephone number already exists!");
        return;
    }
}
For more details, see Validate/Modify User Input Using a Script.  
You can download Adaxes 2011.2 here.