Q.850 cause 28 Invalid number format (address incomplete)

What Q.850 cause 28 means

The digits the system actually sent do not form a dialable number — too few, wrongly prefixed, or containing characters that should not be there. Different from cause 1, where the number is well formed but not live. Very often a dial plan or CRM formatting problem. SIP equivalent is 484 Address Incomplete.

Normal class
Sent by The originating switch, or your own PBX dial plan
Common

What Q.850 cause 28 Invalid number format (address incomplete) actually means

The called number failed a format or completeness check before anyone tried to reach it. The specification notes the condition can be detected either immediately when an end-of-dialling signal arrives, or on a timeout after the last digit received — which is why the same bad number can fail instantly on one system and after a pause on another. Nothing has been said about whether the number exists, because the network never got as far as looking. RFC 3398 maps it to SIP 484 Address Incomplete.

This cause indicates that the called party cannot be reached because the called party number is not in a valid format or is not complete. NOTE – This condition may be determined: – immediately after reception of an end of pulsing (ST) signal; or – on time-out after the last received digit.
— ITU-T Q.850 section 6.2.7.1.22

The SIP equivalent of Q.850 cause 28

A gateway translating this cause into SIP reports it as SIP 484 Address Incomplete. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 28 on a business phone system

Digits missing from the end of the numberA UK number copied from a document with a digit lost, or an extension typed short, arrives incomplete. The switch waits for more, gets nothing and reports the address as unfinished.
Outbound access code problemsWhere a system requires a 9 to reach an outside line, forgetting it sends internal-looking digits to the trunk. The reverse is just as common: a 9 left at the front of a number pasted from elsewhere.
Mixed international and national formatsCombinations like 0044 followed by a leading zero, or a plus sign where the trunk expects 00, produce digit strings that match no valid pattern. Very common in address books built over many years.
Click-to-dial passing unnormalised textCRM records containing spaces, brackets, hyphens or extension suffixes get dialled literally by some integrations. What the screen shows as a tidy number reaches the trunk as something unusable.
A dial plan rule stripping or adding too muchStrip and prepend rules that overlap can remove a digit that should have stayed. The user dials correctly and the system still sends a malformed number, which makes the fault hard to believe until you read the trace.

How to fix Q.850 cause 28

Read the exact digits the system sentTake the called number from the SIP trace or CDR, not from the caller. This one step resolves most cause 28 investigations, because the digits sent are usually not the digits intended.
Standardise on full international format in your dataStore numbers as +44 followed by the national number without the leading zero, everywhere: CRM, address books, website. Ambiguous local formats are the root of most of these failures.
Audit the strip and prepend rulesWork through the outbound routes in order and trace what a typical number becomes at each stage. Look particularly for two rules that both act on the leading zero.
Check the inter-digit timeoutIf users report that calls fail when they dial slowly, the system is deciding they have finished before they have. Lengthening the timeout, or having them press the dial key, resolves it.
Test click-to-dial with a deliberately messy recordPut a number with brackets and spaces into a test CRM record and dial it. If it fails, the integration is not normalising and needs a formatting rule rather than a change to your phone system.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about Q.850 cause 28

Does cause 28 mean the number does not exist?

No. Nobody checked whether it exists, because the digits were rejected before the lookup. The number may be perfectly valid once formatted correctly. If the format is fine but the number is not in service, you get cause 1 instead.

Why do calls dialled from our CRM fail with cause 28?

Because the integration is probably sending the number exactly as it is stored, punctuation and all. Clean the stored data into full international format and add a normalisation rule so future records are dialled correctly regardless of how they are typed.

Why does the same number sometimes work and sometimes fail?

Usually a timing issue. The specification allows the condition to be detected on a timeout after the last digit, so a user who pauses mid-number can trigger it while the same number dialled quickly succeeds. Press the dial or send key to end dialling explicitly.

Related Q.850 cause values

Source. Q.850 cause 28 Invalid number format (address incomplete) is defined in ITU-T Q.850 section 6.2.7.1.22 and registered in the ITU-T Recommendation Q.850 (10/2018). The causes and fixes above are drawn from our own experience supporting UK business phone systems.

All Call hangup causes (Q.850) codes