Had a conversation with a provider the other day and she pointed out the importance hobbyists blocking their caller ID when calling an agency.  She did not realize that this doesn't work the way she thought it did.

I happen to have significant expertise on this issue and thought I should post a quick reminder to people...

Blocking caller ID, either on a call-by-call basis or by blocking it on your line, only prevents your number from being displayed on the box at the far end.  If you are calling a toll free (800, 888, etc.) number, your number will ALWAYS be captured and will ALWAYS appear on their 800 bill whether you block or not.

You are partially correct about the 800 numbers. There are certain exceptions.

If the party on the other end uses a caller id device, or one is embedded in the phone the origin number will display only if ANI (Automatic Number Identification)is passed by the service provider (SBC, ATT, etc.), and in most places it is.

If the phone is located within a building using a PBX or CBX phone switch (Rolm, Siemens, ATT, etc.) the number of the originating call will show only if the Display ANI toggle is turned on.  

This varies from phone switch to phone switch. In the case of older POTS (Plain Old Telephone Systems) using analog rotary switches (mostly in the midwest) a very high percentage will not pass ANI because it is supplied digitally via DTMF (Digital Tone Multi Frequency), and these switches are analog devices.

HPG

Another good idea to keep your number from being traced, yet still have the convenience of using your home, work, or cell phone is to use a pre-paid calling card.  I did this a lot when I worked for agencies and didn't want the client to get my personal number.  It usually works even if someone doesn't accept blocked calls.....

Just my .02

Jaclyn

There are also some PBX systems that will display the number blocked or not. I got numbers from a few providers who were realy unerved when I called them on those numbers.

would you call a lady when you knew she had blocked the #? Perhaps I'm misunderstanding & you didn't know till you had freaked her out.

Lisa

I could be wrong, but I seriously doubt the system displays the number with a "hey, this is a blocked number" message.  It just displays it.  But I'd be careful after the first time it happened and only call on the "public" number I'd been given or seen in an ad.

Again, that's only if the ANI/DTMF is being sent by the Local CO/Service Provider.  Without ANI/DTMF the PBX has no way of knowing the number of origin.

I am not an SS7 expert. I relized this little featue when I called the main office number and asked to be connected to my extension I noticed my cell number was being displayed and I had caller ID block.  What gives?

I am not an SS7 expert. I relized this little featue when I called the main office number and asked to be connected to my extension I noticed my cell number was being displayed and I had caller ID block.  What gives?

Hmmm, that's odd maybe the Local CO or LEC (Central Office or Local Exchange Carrier) is passing ANI/DTMF and not honoring blocked ID's, which isn't too likely.

Or, the Number Blocking isn't being passed between your cell carrier and the land line carrier.  Maybe the information in the data-packet is being stripped as it completes the hardware/software handshake bewteen cell and line line carriers.

Or something else completely.