Are the inter-office dropped calls coming from the other building?
Do you have QOS set up on your router between the buildings?
Are there any firewall settings on the router between the three subnets?
… it doesn’t really sound like a phone system issue as much as a networking issue. If you’ve got a lot of traffic happening, QOS or some other sort of traffic-shaping would need to prioritize voice traffic over regular network traffic.
You’ll need to look at WHY the calls are dropping. The reason will be in /var/log/asterisk/full but almost always this is because something is getting in the way and trying to do SIP ALG. Maybe the phones are trying to say they’re on an external IP Address? There’s a router in the middle that’s messing with SIP traffic?
Unfortunately, you can’t really guess here, you need to actually look 8)
On this VLAN question - I’ve seen lots of people do this, and it almost always ends up with one weird bit of stuff that doesn’t let it work seamlessly. I’ve tried several different VLAN installations myself, and I always end up backing out and either installing a real LAN (drop a new switch in and don’t share the address space + set up routing rules so that the new network can talk to the Internet) or learn to share the address space (use the 10.10.x.x/16 network and put everyone in their own zone).
The frustration about this is that it “almost” works every time, but there’s always one thing that screws with you. Everyone agrees that it should work and would be awesome, but we’ve had lots of posters here (me included) that just can’t get it to light reliably.
Oh, it’s “one thing” for you? Lucky. I guess that’s why you bring in the Big Bucks.
QoS (highly recommended) isn’t the answer since it only kicks in at high load levels – we get bad calls at 40% utilization sometimes, when QoS rules aren’t close to kicking in. VLANs (also highly regarded) don’t help either, as they all come together at the ISP link, plus everything else you alluded to. Bandwidth alone (the standard suggestion) isn’t the answer if Latency & Errors aren’t under control. Our worst outage to date was when a 3rd-party router, 3 hops up the line just stopped talking & there’s no more “Distributed Mesh” to route around it. I keep hoping to find something useful in the “Bufferbloat” conversation, but it’s still nascent & seems to be going nowhere for existing equipment.
I know this will ruffle a lot of feathers, but I still don’t really like VoIP, if only because it steals from my Network. I still remember the days when a phone call would shut off Data, even if the Data were more important. I’m +1000 on the idea of a separate-but-equal Physical Layer for the phones & PBX. But by the time we’ve spent that money we could go back to “old school” PBXes & there would be no need for VoIP in the first place.
Oh, it isn’t just “one thing”, it’s just “one thing at a time” and it’s a different “one thing” every time, and the “one thing” changes while we watch. It’s always tantalizingly close to working, but I just can’t get it to work reliably in my experience.