security by obscurity folks... if nobody knows he has this feature, then it doesn't hurt anything as the door is only unlocked for a short time. so unless somebody happens to be trying to open the apt building door while his phone rings by coincidence, nothing happens. And since it is a VOIP number, it is very hard to tie that to a physical address anyway. So unless somebody got his cell phone, it would be very hard to determine that phone number he used.