TELECOM Digest OnLine - Sorted: VMWI Questions About SDT and FSK

VMWI Questions About SDT and FSK

Bryan Apple (
17 Nov 2004 10:57:36 -0800

I have some questions about the proper implementation for VMWI.
Specifically, I have CallVantage VOIP with a DLINK DVG-1120M, but I
think this issue is somewhat generic. I have posted some messages on
the VOIP forum at, but they don't seem to be a very
technical bunch over there. Maybe this group has some old-time telco
folks that know how things are supposed to work!

In my VOIP environment there are 3 parts to this. The VOIP provider
also offers VoiceMail, and they send status messages to my TA
(telephone adapter) to control the VMWI and SDT. The DLINK TA
provides the FXS interface to my phone, and it generates the FSK and
SDT based on the status as provided my the VOIP provider. Effectively
it acts as the CO or PBX. Finally, the phone must detect the SDT or
FSK and turn on/off the VMWI indicator. I realize that different
phones may do this somewhat differently.

I believe that phones will only go off-hook on their own to check for
SDT under two circumstances: 1) after a call has been completed; 2)
after an un-answered ring. But with FSK I would think the signaling
could be PUSHED down the phone line at any time (as long as the line
is idle).

My problem is that my DLINK TA will only send the FSK to set/clear the
VMWI after the phone has gone through an off-hook->idle cycle. Of
course that would be the same requirement needed to detect SDT, so it
seems to me they have not implemented FSK very well. This becomes
especially problematic when you can change the status remotely - by
dialing into VM from another phone, or accessing via the web,
listening to VM, which then sends the status change to the TA, but the
TA fails to pass that information along via an FSK signal. This seems
wrong to me.

Do I have an accurate understanding of SDT and FSK signaling for VMWI?

Can anybody tell me if other implementations send FSK signaling
independent of a SDT detection event?

What do Telcos that offer VM do?
What about PBX's?
What about other VOIP adapters?

Details of a little test I performed:
1) Call comes in to CallVantage, they notify the TA with CRCX (hd), TA
says OK;

2) No answer, DLCX, TA says OK, RQNT(hd), TA says OK;

3) Caller leaves message and CallVantage notifies TA with RQNT
S=vmwi(+), not sure why but they send this again, TA says OK;

4) I waited for at least 5 min, the TA never put the FSK signal on the
local line to turn on the VMWI;

5) I pick up the local phone, hear the SDT, after hanging up the FSK
signal is put on the line;

6) TA sends NTFY, they say OK, they say RQNT, TA says OK, they say
MDCX with S=mwi, TA says OK;

Same after the VM has been cleared (I deleted it via the web

7) CallVantage sends RQNT S=vmwi(-), TA says OK;

8) I wait at least 5 min, the TA never put the FSK signal on the local
line to turn off the VMWI;

9) I pick up the phone, SDT is gone, after hanging up the FSK signal
is put on the line.


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