You are correct, it does not drop any active calls when doing a reload. But it does make the peers unreachable. And they don’t instantly come back, they only come back when the client recognizes that the registration has expired and they need to re-register. So the phones re-register at random times after the reload, and it usually takes about 15-20 minutes before they’ve all expired and re-registered.
Here’s the general context of my sip.conf file (I left out the individual peers as that would be a security problem if I posted it here, but let me know if there’s something specific you want to see from there):
This is the same configuration I have on my Asterisk 11 server, where this issue doesn’t exist.