It's looks like you are in the process creating the Call Accounting Software .. right?
To communicate with the PBX system, you have to follow the following things:
1. Create the communication serial cable
2. Set the PBX communication setting such as:
speed, parity, databit, stop bit and the line feed
3. use the MSCOMM to receive any string from the PBX's port to the PC's port
4. follow the below codes:

when opening the Comm Port

If MSComm1.PortOpen = False Then
MSComm1.CommPort = 2 'COM2 the Comm port that you are using for example
MSComm1.RThreshold = 1
MSComm1.SThreshold = 1
MSComm1.ParityReplace = "?"
MSComm1.InputLen = 1
MSComm1.EOFEnable = True
MSComm1.RTSEnable = True
MSComm1.Handshaking = comRTSXOnXOff
MSComm1.Settings = ' The setting must same setting as you have setted on your PBX
MSComm1.PortOpen = True
End If



in MSCOMM_Oncomm event

Private Static Sub MSComm1_OnComm()
Dim iRecord
Dim EVMsg$
Dim ERMsg$
' Branch according to the CommEvent property.
Select Case MSComm1.CommEvent
' Event messages.
Case comEvReceive
EVMsg$ = "Receiving"
' This you have to call the procedure that will read any characters
' That received from the PC's port then may be you can do the
' Cost calculation process

Case comEvSend
EVMsg$ = "Sending"
Case comEvCTS
EVMsg$ = "Change in CTS Detected"
Case comEvDSR
EVMsg$ = "Change in DSR Detected"
Case comEvCD
EVMsg$ = "Change in CD Detected"
Case comEvRing
EVMsg$ = "The Phone is Ringing"
Case comEvEOF
EVMsg$ = "End of File Detected"

' Error messages.
Case comBreak
ERMsg$ = "Break Received"
Case comCDTO
ERMsg$ = "Carrier Detect Timeout"
Case comCTSTO
ERMsg$ = "CTS Timeout"
Case comDCB
ERMsg$ = "Error retrieving DCB"
Case comDSRTO
ERMsg$ = "DSR Timeout"
Case comFrame
ERMsg$ = "Framing Error"
Case comOverrun
ERMsg$ = "Overrun Error"
Case comRxOver
ERMsg$ = "Receive Buffer Overflow"
Case comRxParity
ERMsg$ = "Parity Error"
Case comTxFull
ERMsg$ = "Transmit Buffer Full"
Case Else
ERMsg$ = "Unknown error or event"
End Select
End Sub

by the way, what brand or models of PBX that you want to create for the call accounting?..