Field Guide · term

Also known as: private call, individual call, unit call, unit-to-unit call

A private call is a one-to-one, unit-to-unit voice call on a trunked system: instead of addressing a talkgroup, one radio calls a single other radio, and the control channel issues a channel grant naming the two radio IDs rather than a group.1 It is the trunked equivalent of a phone call between two handsets, distinct from the many-to-many group call that most dispatch traffic uses.

control channel grant: 4567 → 8890 (private) radio 4567 radio 8890 both retune, no group
A private call is granted to two radio IDs, not a talkgroup, and only those two radios follow the call.

How it works

A private call begins when one radio requests a connection to a specific target radio ID. On some systems the target’s radio first acknowledges (an alert or “call setup” handshake) before audio flows; on others the call sets up immediately. Either way the trunking controller allocates a voice channel and sends a channel grant that carries two unit addresses — the source and the destination — rather than a single group address. Both radios retune to the granted channel (and timeslot, on TDMA systems), and no other radio in the fleet un-mutes because the call is not addressed to any group they monitor.

The distinguishing feature, from a monitor’s point of view, is the addressing. A group call grant reads “talkgroup 101 → channel 3”; a private call grant reads “unit 4567 → unit 8890 → channel 3”. That difference in the grant’s opcode and fields is exactly what lets a decoder classify the call before any audio is even present.

Variants

  • P25 carries private calls as unit-to-unit voice service messages on the control channel, with the grant naming a source and target Source/Destination Unit ID. A separate answer-request/response handshake can precede the grant.
  • DMR Tier III and Motorola/EF Johnson systems support individual calls the same way, distinguishing them from group calls by the call-type bits in the signalling block (CSBK on DMR).
  • Analog SmartNet/Motorola Type II systems also carry private (interconnect and unit-to-unit) calls, flagged by the command word.

Private calls are frequently short — a supervisor reaching one unit, a status query — and on many systems they can be encrypted end-to-end even when group traffic is in the clear.

In practice

The setup handshake is what distinguishes the two common private-call styles. In an availability-checked private call, the system first pages the target unit and waits for it to answer before allocating a voice channel; if the target does not respond, the call is never granted, so a monitor may see the request and the page but no grant. In a direct private call, the system grants a channel immediately and the target simply un-mutes when it hears its own ID, at the cost of possibly keying up a channel for a unit that is switched off. Systems also differ in whether a private call ties up a full voice channel for two units — an inefficient use of a shared resource — which is one reason some agencies restrict or disable the feature and route one-to-one traffic through telephone interconnect or data messaging instead.

For a listener, the practical consequence is that private calls are sparse, bursty, and easy to miss: they are not tied to a talkgroup on any scan list, they can be brief, and they may be encrypted. Catching them at all depends on parsing the private-call opcodes rather than only following group traffic, which is why a trunk-tracking scanner treats them as a distinct call class.

Relevance to SDR

For a scanner, private calls matter because they are the traffic that isn’t on the talkgroup list. A monitor watching only group calls will miss unit-to-unit conversations entirely unless it also parses private-call grants. GopherTrunk reads the grant opcodes on the control channel and classifies each call as group, private, or emergency; when it sees a private-call grant it can task a receiver to the assigned channel and log both radio IDs involved, populating its unit activity view with who called whom. This is the same mechanism it uses for group calls — follow the grant, tune the voice channel — but the metadata recovered is a pair of units rather than a group.

As always, GopherTrunk decodes what is transmitted in the clear or with recoverable scrambling; a private call that is encrypted end-to-end will still be detected and logged (the grant is unencrypted control-channel signalling), but its audio will not be intelligible. Real systems that carry private calls include P25 Phase 1 and Phase 2, DMR Tier III, and legacy Motorola trunking.

Sources

  1. Trunked radio system — Wikipedia, on unit-to-unit (private/individual) calls versus group calls on trunked systems. ↩

See also