Calls¶

Plain telephony needs only standard DSS1 plus the registration reply. Bytes below are I-frame information fields (layer 3), TEI 64.

Incoming call (network to phone)¶

sequenceDiagram
  participant N as PBX
  participant P as Phone
  N->>P: SETUP (flag 0)
  P->>N: ALERTING (flag 1)
  Note over P: phone rings, user lifts handset
  P->>N: CONNECT
  N->>P: CONNECT ACKNOWLEDGE
  Note over N,P: speech on B1
  N->>P: DISCONNECT (cause 16)
  P->>N: RELEASE
  N->>P: RELEASE COMPLETE

SETUP can be sent as an I-frame or as a broadcast UI frame (SAPI 0, TEI 127); the phone accepts both. ✅ Tested.

08 01 01 05                       SETUP, call reference 1, flag 0
04 03 80 90 a3                    bearer capability: speech, A-law
18 01 81                          channel: B1 (preferred)
28 09 "Test call"                 display text (optional)
6c <len> 21 80 "0301234"          calling party number
70 <len> 81 "1"                   called party number

The phone answers 08 01 81 01 (ALERTING, flag 1) and, when the handset is lifted, 08 01 81 07 (CONNECT). The network sends 08 01 01 0f (CONNECT ACKNOWLEDGE).

To clear from the network send DISCONNECT with a cause, flag 0:

08 01 01 45 08 02 80 90

The phone returns RELEASE (08 01 81 4d) or RELEASE COMPLETE. If the network sets the flag wrong (81), the phone answers RELEASE COMPLETE with cause 81 and the call is not cleared. ✅ Tested.

Outgoing call (phone to network)¶

sequenceDiagram
  participant P as Phone
  participant N as PBX
  Note over P: user lifts handset
  P->>N: SETUP (no called number)
  N->>P: SETUP ACKNOWLEDGE
  loop each key press
    P->>N: INFORMATION (one digit)
  end
  Note over N: number complete, route the call
  P->>N: DISCONNECT (user hangs up)
  N->>P: RELEASE
  P->>N: RELEASE COMPLETE

Lifting the handset sends SETUP with no called party number, so the network must support overlap sending. ✅ Tested:

08 01 01 05
04 03 80 90 a3        bearer capability: speech, A-law
18 01 83              channel: any
7d 02 91 81           high layer compatibility: telephony
7e 01 04              user-user

The network answers SETUP ACKNOWLEDGE with the call reference flag set:

08 01 81 0d 18 01 89  channel: B1 exclusive

Each key press then arrives as its own INFORMATION message with one digit, flag 0:

08 01 01 7b 70 02 81 31      digit "1"
08 01 01 7b 70 02 81 32      digit "2"

The digits appear on the phone's display as they are dialled. The phone does not send a "number complete" marker: the network must decide when dialling is finished (digit timeout, a known numbering plan, or #).

When the user hangs up the phone sends 08 01 01 45 08 02 80 90 (DISCONNECT); the network answers RELEASE and the phone sends RELEASE COMPLETE. ✅ Tested.

After dialling (not yet tested)¶

From the phone's call-state tables (📖 Read): in the overlap-sending state it accepts ALERTING (moving to "call delivered"), CALL PROCEEDING (moving to "outgoing proceeding") and CONNECT (moving to "active"; it answers with CONNECT ACKNOWLEDGE). So the network should answer a completed number with CALL PROCEEDING and then ALERTING and CONNECT as the far end progresses. The B-channel audio itself is not part of the D-channel and was not tested.