Yes, if you already subscribe to Sky Broadband/Talk all you need to do is buy a handset and plug it in. By all means post back if you encounter any problems.
Do be aware that business use of Sky Talk/Broadband is specifically excluded in the terms and conditions you've signed up to. It means that if there's a fault you won't get any priority when it comes to being fixed, and Sky won't accept financial liability for any issues arising from loss of service etc.
