Well remote sip phones with SBC or otherwise requires a session manager for the sets to register to , if you already know this apologies for teaching you how to suck eggs , my preference to you would be to get an avaya dev connect document and implement based on the technologies that have been tested, as really if you have home devices trying to act in an business critical environment , it does not matter if you are using SIP, H.323, or the force..... the weak point is always the homeworkers edge device.
Get that correct and all will work ok , granted that may mean purchasing some dedicated hardware for the homeworkers and a business connection .... but hey if its critical to needs , the amount of time you will spend farting about in man hours implementing and supporting the headache will be minimal ........ the other way forward is to have a true site to home VPN (again dedicated connection preferred ), and simply static the phones and have them act as a normal endpoint , you need to static the devices at each home location (eggs again!) , but i have never had any issues using this method.
ACSS (UC/SBCE/SM/SME)
Not that they mean a thing anymore , get a brain dump pass the test crash the system.