You're using Digium phones?
The visual voicemail component of those phones is indeed SIP-based. I'm not sure what the actual media transport is but I'd wager it's simply RTP.
I've started using Digium phones pretty extensively lately and there are some quirks and major bugs with some versions of the DPMA module. Could you expand on what specific issues you're having?
Are the apps just plain not functioning or are you seeing quality issues with the voicemails as they play?
Are the phones and the PBX in the same VLAN?
The visual voicemail transport is rtp. The initiation of the server delivering the voicemail to the phone and phone playing it seems to involve several packets of TCPIP and TLS. What we are experiencing is that when we choose messages, choose a voicemail and push play, it will say fetching voicemail for a second or two, then the progress bar will sart moving but no audio for anywhere from .5 seconds to 15 seconds. Then near the end, itncus the ending of the voicemail off because I assume it assumes its reached end of the voicemail.
In the packet traces, it looks like a bunch of tcpip and TLS packets, then rtp for about 20 packets (I assume the dead air audio), then a tcp reset is sent from the phone to the switchvox, then a much longer rtp stream (which I assume is the actual audio playing) then it's done.
I tested with a single phone hooked to a small physically separate switch and the switchvox. Voicemails seem to play ok until we played about 6 voicemails then started delaying a bit again. In packet capture, every voicemail we played showed a tcp reset packet, even ones that seem to play fine, which left me a bit confused
As far as topology, the server itself is in a different vlan than the phones. We are actually utilizing one server with phones spread through several offices running 20mb/s fiber. Each location on its own vlan subnet. This is all connected via Cisco switches from our carrier provider and gives us average of 8ms latency to all office locations.
When we did initial testing, the system worked fine, no issues until we actually went live and added the phones to each of the offices, the delays got worse and worse, even though each location is again on its own vlan.