Requesting indepth information on how Centrix line work at the CO level

[datajockey123]

Hello all
I need some extensive background information on how Centrix lines work and how they are handled by the CO switch (DMS 500) at the providers end.
I have a client that has an ongoing 'toll fraud' issue and the billing information supplied to us by the service provider doesn't add up/ make sense.
Looking for someone who is a CO engineer or someone that has worked on the frame possibly to give more information on how multiple calls on one phone line can take place at the same time.
In advance, many thanks.
Look forward to chatting
Datajockey123

 

[curlycord]

Which voice mail system is it?
Maybe they have a DID centrex line with multiple dial out channels.
Think like a PRI, you have the main DID number and everyone dial's 9 to grab a channel so everything looks as if it goes out the same phone number.





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[ucxguy]

You can have multiple calls "on a single CO line" when a CO based transfer or call forward feature is enabled on the line. As soon as the transfer or forward is completed, the line is available for another call. The CO bills you for the duration of each call to the transfer/forward destination.

For example, let's say a receptionist has a single CO line. She answers a call at 9:15 AM and transfers the call to destination 1. The CO performs the transfer and the CO line becomes free. At 9:16 AM, the receptionist answers another call and transfers the call to destination 2. The CO performs the transfer and the CO line is free again. The duration of the first transferred call is 20 minutes, the duration of the second call is 5 minutes. You will be billed for 20 minutes to destination 1 and for 5 minutes to destination 2 even though the CO line was actually active for less than 2 minutes.

 

[ucxguy]

With regard to the toll fraud, an attacker could use a scenario similar to
1. The attacker hacks a voicemail account
2. The attacker configures the mailbox to initiate a CO based transfer to an external destination of their choice
3. An incoming call is placed to the CO line and directed to the compromised mailbox
4. The call gets answered by the compromised voicemail account
5. A voicemail command is used to initiate the transfer
6. The voicemail system initiates the transfer by dialing the CO transfer code and the external destination
7. CO receives the digits to invoke the transfer and the destination digits, so it removes the call from the line and redirects the call to the specified external destination
8. The CO line is free for another call
The same scenario can be repeated again and again. When the attacker is somewhat sophisticated and uses a computer and scripting, it can take as little as a few seconds to get through the steps 2 - 6. The attacker can make as many "concurrent" calls using the same CO line as the CO will allow. When done, the attacker can also restore the original configuration of the mailbox to make it appear normal again.