Private Lines
About Private Line

Private Line covers what has occurred, is occurring, and will ocurr in telecommunications. Since communication technology constantly changes, you can expect new content posted regularly.

Consider this site an authoritative resource. Its moderators have successful careers in the telecommunications industry. Utilize the content and send comments. As a site about communicating, conversation is encouraged.


Thomas Farely

Tom has produced since 1995. He is now a freelance technology writer who contributes regularly to the site.

His knowledge of telecommunications has served, most notably, the American Heritage Invention and Technology Magazine and The History Channel.
His interview on Alexander Graham Bell will air on the History Channel the end of 2006.

Ken Schmidt

Ken is a licensed attorney who has worked in the tower industry for seven years. He has managed the development of broadcast towers nationwide and developed and built cell towers.

He has been quoted in newspapers and magazines on issues regarding cell towers and has spoke at industry and non-industry conferences on cell tower related issues.

He is recognized as an expert on cell tower leases and due diligence processes for tower acquisitions.

« London Exchange Names in 1916 | | Party Line Histroy »

January 03, 2006

Posted by Tom Farley & Mark van der Hoek at 11:30 PM

Mobile Prefixes

Editor's note. This page discusses early mobile telephone prefixes. If in fact they existed. The chart Geoff refers is linked to below, a Bell System list of approved prefix numbers and names:

Hi Tom-

Yes, that chart appears to be correct.

As you know, until IMTS came around, mobile numbers had no direct dialing capability and only 5 digit numbers, so a mobile number was always prefixed by its home channel, thus:

JL 5-5575 or YK 4-3378

To reach a mobile phone, you had to dial the operator and then ask for the mobile operator in the city of registry of the mobile phone. Then you had to ask the mobile operator to ring the mobile, by telling her the mobile number, such as " JL 5-5575 , city of registry Los Angeles." A few years before Pac Tel switched over to IMTS, they were telling people to start using the area code rather than the city of registry. So they were expecting you to ask for "Area Code 213, JL 5-5575." The mobile terminalwould out-pulse 5 digits, i.e. 5 5575. Theoretically that could ring more than one phone, if a foreign mobile with the same number were in that area. In other words, an Alabama mobile with JL 5-5575 would also ring as there was no way to identify the city of registry with a 5 digit system. IMTS, as you know, was a 7 digit system. The IMTS phone was programmed with the area code then the last 4 digits. The prefix was not part of the ANI string. In other words, my mobile number of (408) 679-5575 was programmed into my mobile as 408-5575. As a hacker, I had a devil of a time figuring that out when I was experimenting. Even when I signed up for service, Pac Tel never said a thing about it.

There were 11 channels, as identified on the channel buttons of your Livermore attache phone. Each would originally have been a mobile # prefix. I see 57 is the numeric equivalent of channel prefix JR, but also JS and JP. Looking at the chart, I see 97 is also reserved, and would represent channels YR or YS or YP. The VHF channels start with a J or a Y so you would also need to reserve prefixes 55 and 95.

But I don't see any point in ever having reserved those exchange prefixes for mobiles, since there was no way they could ever have been dialed direct in an MTS manual system. You always had to ask for a mobile operator, and then say the channel, rather than the prefix. I think the reservation of those exchange prefixes was part of an early idea that someday they would have direct land to mobile dialing even in MTS phone systems, which never happened.

On another more arcane subject, those decoder gear wheels in the earliest mobile phones were not capable of using certain number combinations by reason of mathematics, and there was a whole list of numbers to be avoided for various reasons, mainly because the wheel did not revert to the resting position as soon as it detected a mismatch, as the transistorized selectors did. It only reverted to rest when the digits from the phone company transmitter ceased and a mismatch occurred. Thus, if you picked the wrong number to use, the phone could ring when someone else was dialed. That must have been very confusing. In other words, there were some numbers which added up to the same number of ratchets of the wheel such that the bell would ring. Also, the number selected couldn't add up to a total larger than a certain figure. This limited the available number pool.



The Telephone Name EXchange Project by Robert Crowe (click here to go there)

IMTS or Improved Mobile Telephone History Links below:


Geoff is an ardent mobile radio enthusiast, please visit his site soon.

More IMTS madness? Of course. Take a look at a company newsletter describing the 1982 cutover in Pac Bell land:

Page One / Page Two / Page Three / Page Four

Post a comment

(If you haven't left a comment here before, you may need to be approved by the site owner before your comment will appear. Until then, it won't appear on the entry. Thanks for waiting.)

Human Verification:

Article Index

Recent Posts

Powered by
Movable Type 3.2