What MMI Error Messages Mean on iPhone: How It Works

How to read the replies an iPhone gives after dialling a service code. This page explains what it does, how to use it on an iPhone and which carriers it applies to.

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What is What MMI Error Messages Mean?

Dialling a supplementary service string produces one of a small set of responses, and knowing which is which saves a lot of pointless retrying.

A connection problem or invalid MMI code message almost never means the code was typed wrongly. It means the string left the phone, reached the network, and came back unrecognised or rejected. The commonest cause by far is that the service is not provisioned on your subscription, which is exactly the situation for call barring on most US consumer lines. Poor signal, a line that has not finished registering, and being on Wi-Fi calling with no cellular registration will produce the same wording.

Setting interrogation succeeded means the query worked and the detail that follows is the real answer, even when that detail is simply that the service is not active. Setting activation failed means the network understood the request and refused it, which is a more specific and more useful failure than an invalid code message. A request that hangs on please wait and then times out usually means the network never replied at all.

Two practical rules. Dial these strings from the Phone keypad with a cellular connection rather than over Wi-Fi calling alone, and if a string fails once with an invalid code message, retrying it identically will not help. Try the read only interrogation form of the same service instead, because if the interrogation errors too, the service does not exist on your line.

How do I do it?

1. Confirm the iPhone has a cellular bar or two rather than Wi-Fi calling only.
2. Try the interrogation form of the service, which starts *# and needs no password.
3. If the interrogation also errors, the service is not provisioned and no variation of the string will work.
4. Contact the carrier and ask specifically whether the supplementary service is provisioned on the line.

Which carriers does this apply to?

Published for	AT&T, T-Mobile and any iPhone
Type	Works on all iPhones
Good to know	Reference material rather than a dialable code. The exact wording varies between iOS versions, but the categories of response are consistent.

Related call barring and sim pin codes

• *#591#: Check Multiple Subscriber Profile
• **052*12345678*1234*1234#: Unblock PIN2 With the PUK2
• **05*12345678*1234*1234#: Unblock a SIM With the PUK
• **042*0000*1234*1234#: Change Your SIM PIN2
• **04*0000*1234*1234#: Change Your SIM PIN
• #43*11#: Turn Off Call Waiting for Voice Only

See all call barring and sim pin codes or every category of iPhone codes.

How Secret Codes helps

Secret Codes keeps this and every other entry a search away. Pick your carrier once and each entry says whether it is published for your line. You can save favourites, add a private note, and record whether a code worked on your line.