All About BLV

Verification and emergency interrupts are two operator functions that have always fascinated the phone phreak world.  Here then is an explanation of just how it all really works.  (Note: this article is written solely on the AT&T TSPS process of verification.)

Let's say Smith needs to get ahold of his friend, Jones.  Jones' telephone line is busy, and Smith must talk to Jones immediately.  He calls the operator, by dialing 00 for an AT&T TSPS operator (or in some areas, 0 still gets TSPS).  The operator answers, and asks if she can help him.  Smith replies that he needs to interrupt a call in progress so he can get through.  He tells the operator Jones' number.  After a few seconds, he is connected to Jones and they talk.

The name for this process is Busy Line Verification, or BLV.  BLV is the telco term for this process, but it has been called "Verification," "Autoverify," "Emergency Interrupt," "Break into a line," "REMOB," and others.  BLV is the result of a TSPS that uses a Stored Program Control System (SPCS) called the Generic 9 program.  Before the rise of TSPS in 1969, cordboard operators did the verification process.  The introduction of BLV via TSPS brought about more operator security features.  The Generic 9 SPCS and hardware was first installed in Tucson, Dayton, and Columbus, Ohio in 1979: By now virtually every TSPS has the Generic 9 program.

A TSPS operator does the actual verification.  If Jones was in the 314 area code and Smith was in the 815 area code, Smith would dial 00 to reach a TSPS that served him.  Now, Smith, the customer, would tell the operator he needed an emergency interrupt on a given number: 314-555-1212

The 815 TSPS operator who answered Smith's call cannot do the interrupt outside of her own area code, (her service area), so she would call an Inward Operator for Jones' area code, 314, with KP+314+TTC+121+ST, where TTC is an optional Terminating Toll Center code that is necessary in some areas.

Now a TSPS operator in the 314 area code would receive the 815 TSPS operator's call, but a lamp on the 314 operator's console would tell her she was being reached with an inward routing.  The 815 operator then would say something along the lines of she needed an interrupt on 314-555-1212, and her customers name was J. Smith.

The 314 Inward (which is really a TSPS) would then dial Jones' number, in a normal Direct-Distance Dialing (DDD) fashion.  (DDD by an operator is really called ODDD, for Operator Direct-Distance Dialing.)  If the line was not busy, then the 314 Inward would report this to the 815 TSPS, who would then report to the customer (Smith) that 314-555-1212 was not busy and he could call as normal.  However, if the given number (in this case, 314-555-1212) was busy, then the process of an Emergency Interrupt would begin.

The 314 Inward would seize a verification trunk (or BLV trunk) to the toll office that served the local loop of the requested number (555-1212).  A feature of the TSPS checks the line asked to be verified against a list of lines that should not be verified, such as radio station call-in lines, police station lines, etc.  If the line number a customer gives is on this software list, then the verification cannot be done, and the operator notifies the customer.

The 314 Inward would then press her VFY (VeriFY) key on her TSPS console, and the equipment would outpulse (onto the BLV trunk) KP+0XX+NXX+XXXX+ST

The KP signal prepares the trunk to accept MF tones, the 0XX is a "screening code" to protect against trunk mismatching, the NXX is the exchange or prefix of the requested number (555), the XXXX is the last four digits of the requested number (1212), and the ST is the STart signal which tells the verification trunk that no more MF digits follow.

The screening code is there to keep a normal Toll Network (used in regular calls) trunk from accidentally connecting to a verification trunk.  If this screening code wasn't present, and a trunk mismatch did occur, someone calling a friend in the same area code might just happen to be connected to his friend's line, and find himself in the middle of a conversation.  But the verification trunk is waiting for an 0XX sequence, and a normal call on a Toll Network trunk does not outpulse an 0XX first.

(Example:  You live at 914-555-1000 and wish to call 914-666-0000.  The routing for your call would be KP+666+0000+ST.  The BLV trunk cannot accept a 666 in place of the proper 0XX routing, and thus would give the caller a re-order tone.)

Also, note that the outpulsing sequence onto a BLV trunk cannot contain an area code.  This is the reason why if a customer requests an interrupt outside of his own NPA, the TSPS operator must call an Inward for the area code that can outpulse onto the proper trunk.

If a TSPS in 815 tried to do an interrupt on a trunk in 314, it would not work.  This proves that there is a BLV network for each NPA, and if you somehow gained access to a BLV trunk, you could only use it for interrupts within the NPA that the trunk was located in.

BLV trunks "hunt" to find the correct trunks to the right Class-5 end office that serves the given local loop.  The same outpulsing sequence is passed along BLV trunks until the trunk serving the toll office that serves the given end office is found.

There is usually one BLV trunk per 10,000 lines (exchange).  So, if a toll office served ten central offices, that toll office would have ten BLV trunks running from a TSPS site to that toll office.

Scrambling the Audio

The operator (in using the VFY key) can hear what is going on on the line (modem, voice, or a dial tone, indicating a phone off-hook), but in a scrambled state.

A speech scrambler circuit within the operator console generates a scramble on the line while the operator is doing a VFY.  The scramble is there to keep operators from listening in on people, but it is not enough to keep an operator from being able to tell if a conversation, modem signal, or a dial tone is present upon the line.

If the operator hears a dial tone, she can only report back to the customer that either the phone is off-hook, or there is a problem with the line, and she can't do anything about it.  This speech scrambling feature is located in the TSPS console, and not on verification trunks.

In the case of Jones and Smith, the 314 Inward would tell the 815 TSPS, and the 815 TSPS would tell the customer.  If there is a conversation on line, the operator presses a key marked EMER INT (EMERgency INTerrupt) on her console.  This causes the operator to be added into a three-way port on the busy line.  The EMER INT key also deactivates the speech scrambling circuit and activates an alerting tone that can be heard by the called customer every 10 seconds.

This tone tells the customer that an operator is on the line.  Some areas don't have the alerting tone, however.

Now, the operator would say "Is this NXX-XXXX?" where NXX-XXXX would be the prefix and suffix of the number that the original customer requesting the interrupt gave the original TSPS.  The customer would confirm the operator had the correct line.  Then the operator would say, "You have a call waiting from (customer name).  Will you accept?"  This gives the customer the chance to say "yes" and let the calling party be connected to him, while the previous party would be disconnected.

If the called customer says "no," then the operator tells the person who requested the interrupt that the called customer would not accept.  The operator can just inform the busy party that someone needed to contact him or her, and have him/her hang up, and then notify the requesting customer that the line is free.  Or, the operator can connect the calling party and the interrupted party without loss of connection.

If a customer requested an interrupt upon a line within his Home NPA (HNPA), then the original answering TSPS operator would do the entire verification process as described above.

The charges for this service (in my area at least) run $1.00 for asking the operator to interrupt a phone call so you can get through.  There is an 80 cent charge if you ask the operator to verify whether the phone you're trying to reach is busy because of a service problem or because of a conversation.  If the line has no conversation on it, there will be no charge for the verification.

The Aftermath

When the customer who initiated the emergency interrupt gets his telephone bill, the charges for the interrupt call will look similar to this:

12-1 530P INTERRUPT CL 314 555 1212 OD 1 1.00

• 12-1 is December First of the current year.
• 530P is the time the call was made to the operator requesting an interrupt.
• INTERRUPT CL is what took place, that is, an interrupt call.
• 314 555 1212 is the number requested.
• OD stands for Operator assisted, Daytime call.
• 1 is the length of the call (in minutes).
• 1.00 is the charge for the interrupt.

The format may be different, depending upon your area and telephone company.

Verification seems to be on a closed network, only accessible by the TSPS.  However, there have been claims of people doing BLV's with Blue Boxes.

I don't know how to accomplish BLV without the assistance of an operator, nor do I know if it can be done.  But hopefully this article has helped people understand how an operator does Busy Line Verification and Emergency Interrupts.