We don't actually support the Shared Line Appearance (SLA) feature on the BCM, although we have a similar feature we refer to as Shared Call Appearance. To have an SCA key appear on a phone without it ringing, you simply exclude it from the ring group for the SCA calls. One caveat is that you cannot answer a ringing SCA key if you are not in the ring group. You can however, pick up a held SCA call if someone else answers it and places it on hold.
The scenario might be where you have one person (the receptionist) answering all the calls, but everyone wants to have the lines appear on their phones. So the receptionist answers the call and placers it on hold, then pages that their is a call on line two. Anyone with an SCA key on their phone can then pick up the call, put it back on hold, transfer it, etc.