CRM 2015: How do I get OrganizationService in a plugin registered in sandbox

I'm new to CRM Online 2015. I wrote a plugin registered in isolationmode = sandbox, update of account. It should save a  typed-in telephonenumber into a custom field in a normalized form. Before this is done, the plugin should read some configurations which are saved in the root businessunit. So i need to receive the entity businessunit with a retrieve Request. Unfortunately when i debug the code organizationService is null. Is it possible to get the IOrganizationService or it is also one of the restrictions of sandbox mode?

public void Execute(IServiceProvider serviceProvider){ 
	var organizationService = ExtractService<IOrganizationService>(serviceProvider);
	... 
	var businessunit = organizationService.Retrieve(...)
}
private static T ExtractService<T>(IServiceProvider serviceProvider){  
	var typeOfService = typeof (T);   
	var service = (T) serviceProvider.GetService(typeOfService);   
	return service; 
}