Using the Motorola i355 as a Walkie-Talkie

You may remember the iconic Motorola i355 (and phones like it) as the bricks that people engaged in manual labor carried around a decade ago.  They were built like tanks and always were making that distinctive “chirp”.  Well, these phones were built so well that many still work just fine today, which is great, because they still have a useful purpose.  While Sprint (Nextel) shutdown the iDEN network back on June 30th of 2013, these phones included a feature called Direct Talk, which was an off-network walkie-talkie function.

Device / Feature Background:

The Motorola i355 (and many other iDEN devices of the era), while utilizing the Direct Talk function, operates within the unlicensed ISM band occupying the 902-928 MHz frequencies.  The actual Motorola term for the product is MOTO Talk, but Sprint/Nextel marketed it as Direct Talk, which is what most people recognize it by.  The technology employs FSK modulation, FHSS (frequency-hopping spread spectrum), and FEC (forward error correction) to setup a digital “call” between devices.  You have the choice of using channels and codes to setup a talkgroup, or to make a private call via a number that is programmed into the phone.  While utilizing the channel/code method, your call is rather insecure, as anyone else in the area that is on the same channel can hear your conversation simply by setting the code to “all”.  However, when utilizing the private call function, your call is rather secure, as someone would have to know the number programmed into the destination phone AND would have to program their phone with the same number in order to receive your call.  The chances of that occurring are rather minimal.

The range of these phones utilizing Direct Talk varies with terrain and surroundings.  People have reported from as little as 1/4 mile (in dense urban areas) all the way up to almost 10 miles (when communicating from a hilltop to a visible low point).  Part of the equation is the antenna in use, and luckily, there is an upgrade available for the i355 (as well as external antenna options).  Due to the digital nature of the function, if you exceed the range of the phone you are trying to contact, you will simply get a message that the other phone cannot be reached.  This is convenient, as you know whether or not the other recipient is hearing what you are saying.

What you need to get started:

You can typically find an i355 on eBay for a reasonable price.  Many of these phones will not be cosmetically as nice as you’d expect a newer phone to be due to the rough nature of the environments they were used in.  However, do not let that deter you; these things are almost indestructible.  As long as it powers on and no ports are physically damaged, you should be good to go.

You’ll likely want to look for a replacement battery right from the start, as the most recent production of this phone was over a decade ago.  You can often find “new” batteries on eBay for a reasonable price.  They’ll likely be old too, but unused, so they should have some life left in them.  Alternatively, you can find a few sellers on eBay (and elsewhere) that sell actual new batteries for the phone.  Using these could be a blessing or a curse; they could be much better than older batteries, but they also could lack manufacturing controls that could damage your device.  YMMV.

You’ll also need to ensure you have a charger for the phone, likely both a home charger and a car charger.  This phone pre-dates standard USB charging connectors, so you won’t be able to use any other charger to charge this phone than an older dedicated Motorola-style charger.

If you want to utilize the private call function (which I highly recommend), you will need to obtain a USED Nextel SIM card.  I have used the 64K / 600 Contact SIM cards with success, but others may work.  This is only necessary so that the phone can fully boot; the network that these phones used to used is long decommissioned.

Finally, you’ll want to upgrade the antenna to maximize the range available between phones.  Find yourself a NNTN5539A or NNTN7510A antenna on eBay.  This antenna is a direct replacement for the pull-out stubby antenna that comes with the phone from the factory.

Getting started:

Once you’ve assembled the above parts, start by inserting the SIM into the phone and powering it up.  I recommend doing a factory reset immediately in order to clear out any old settings from the previous user.  This option can be found in the menu tree by selecting Menu > Settings > Advanced > Reset Defaults > Reset All.  If you are prompted for a PIN, it should be the factory default of 000000.  Additionally, I recommend deleting all contacts that are on the SIM card.  Unfortunately, I’m unaware of any way to bulk-delete these; deleting them one-by-one seems to be the only way.

Next, you want to configure Direct Talk to start when the phone boots (since the phone is useless for actual phone calls without a network).  To do so, select Menu > Settings > Personalize > Power Up > DirecTalk.  Now, restart the phone.  Once the phone begins to boot, you will eventually come to the DirecTalk options screen (you can get there faster by pressing End when it starts to boot).  Next, you’ll be dumped into the DT Options menu.  Select Setup > Direct Launch > Yes.  Restart your phone once more.  This time (and every time going forward), you’ll be able to select the right option key to disallow the phone portion, and then the left option key to enter Direct Talk mode.

Utilizing the Channel/Code service:

Coming Soon

Utilizing the Private Calling function:

Coming Soon

Important things to keep in mind:

Technically, nothing in this setup is truly “secure”.  Anyone that can guess your Channel, Code, and Private Number (if applicable) can listen to anything you are saying.  Now, it is pretty unlikely to encounter someone else using the i355, and even more unlikely that they’ll get all of the above right.  I’d consider this pretty strong security through obscurity.  (Also, be aware that since your calls are not encrypted, people and/or agencies with the right equipment to follow the frequency-hopping can listen in on your conversation as well.)

Useful Part Numbers:

Part NumberDescriptionNotesLinks
NNTN6112Battery Door, Maximum Capacity
NNTN6111Battery Door, Slim
SNN5705CBattery, High Performance 3.6V Lithium (850 mAh)
SNN5705DBattery, High Performance 3.6V Lithium (1140 mAh)
SNN5706BBattery, Maximum Capacity 3.6V Lithium (1050 or 1140 mAh)Requires NNTN6112
NNTN4655Battery, Maximum Capacity 3.6V Lithium (1450 mAh)Requires NNTN6112
NNTN4846CCar Kit, BaseRequires NNTN4998BManual
NNTN4998BCar Kit, CradleRequires NNTN4846C
NNTN4679A / NNTN5165 / NNTN6343BCharger, Car
NNTN5749Charger, Car (Rugged)
NNTN4680A / NNTN4963B / NNTN6257ACharger, Rapid Travel
NNTN6257Charger, Reduced-Sized Wall
NNTN4841Charger, Wall
NNTN5225PACharging Dock, Battery Only
NTN2103 (A/B)Charging Dock, Phone + Battery
NKN6560Data Cable, RS-232
NNTN5406BData Cable, RS-232 with Charging
NNTN6005AData Cable, USB
NNTN5405BData Cable, USB with Charging
NNTN5539A / NNTN7510AExtended Antenna
NNTN4744Hands-Free Car Kit
NNTN5209CiDEN Multi-Unit Charging Station
NNTN5208Microphone, Heavy Duty Remote Speaker
NSN6066Microphone, Remote Speaker
NNTN5330BPTT Headset, Earbud
NNTN5211BPTT Headset, Surveillance
NNTN6312APTT Headset, Surveillance, 3 Wire
NNTN5210 / NNTN5907Swivel Belt Clip Holster
354008Wilson Antenna Adapter (for use with external antennas)Does NOT work with Direct Talk

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