Calls going direct to Voicemail - Explained

[Hands0n]

I found this interesting little piece of information on the giffgaff forum and thought that it might be of interest to anyone afflicted with this problem.

Although the article discusses the experience on an iPhone the circumstances and causes of this happening will manifest on other smartphones too. So, if you are finding calls going to Voicemail while you [or an app] are using mobile data then this is most probably why.

What can you actually do about it? Well, in truth there is nothing that you can do other than change network operator to one that supports NOM001.

Vodafone currently are a NOM002 operator, that is they give priority to data, according to the information below. And so, if this information is entirely accurate, you could expect to have calls go to Voicemail in the circumstances described in the article.





So a lot of people have been commenting in the Help section, asking why when using some data the iPhone goes straight to answer phone. I'm going to explain why this is.

The iPhone cannot simultaneously use EDGE and voice services. That is, if you are on a call, you cannot concurrently access EDGE-data functions.

There is more than one possible mode a GPRS/EDGE phone network can be configured to operate in, NOM001 and NOM002 being the most common. In mode NOM001 if an incoming call occurs during data use the data session will be terminated and the call will come through. In mode NOM002, the call will go straight to voicemail. Yes, O2 therefore giffgaff use mode NOM002.

It's possible to use the iPhone's field test mode to see what Network Operating Mode is for the network - to do this, first disable 3G, then go into field test mode by dialing *3001#12345#* from the phone app keypad then pressing call, then go to GSM Cell Environment -> GPRS Information, and scroll down to "Network Operating Mode".

Not only can this be caused by actively using an app such as safari, (or leaving a data using app active with the screen locked) but background mail checks and push notifications will cause it as well. So if your phone is lying there unattended and it happens to do it's 30 minute (or whatever) mail check just before someone tries to call you - missed call.

Having push notifications enabled is even worse, because it tries to keep an active data session as much as possible, and re-establishes this connection every time the data connection is interrupted.

One good example of this is if you have push notifications active, try making or receiving a call, hang up, and call the phone back in less than 30 seconds or so of the previous call - missed call.

The reason is that the GPRS/EDGE connection was interrupted by the first call, so when the call ended the phone immediately tries to re-establish the push data connection, and on a slow connection this takes up to a minute to do and then go idle before the phone can receive another incoming call.

NOM = Network Operation Mode

*NOM 001 - Voice higher priority than Data
*NOM 002 - Data higher priority than Voice


I hope this helps clear up any questions regarding iPhone's going straight to answer phone.

My only tip could be if you are getting missed calls, disable the push notifications feature as this is the biggest culprit!

Source: http://community.giffgaff.com/t5/Learn-giffgaff-Top-Tips/iPhone-going-straight-to-answer-phone-Theres-a-reason-for-that/td-p/430933

[Ben]

Seems entirely pointless giving data priority on GSM networks when GSM networks aren't exactly designed for data by modern standards.

[DBMandrake]

That information from the giffgaff forum is not entirely correct, or at least incomplete.

The NOM 001 vs NOM 002 mode is technically correct, however in practice nearly all GSM networks in the world use NOM 002, because NOM 001 is not compatible with some (older) handsets, and has some other drawbacks.

So basically, active data sessions on 2G blocking incoming calls (sending them to voicemail etc) is normal for the vast majority of GSM networks around the world, unfortunately. Factor in Smart Phones frequently accessing data in the background to check mail, receive push notifications etc, and you can easily see how some calls are missed and go straight to voicemail.

Vodafone UK on the other hand support a relatively recent GSM feature called DTM or "Dual Transfer Mode", which on modern handsets equipped with DTM support allows for simultaneous data and voice on 2G just like with UMTS. Very few phones support it at this time, (a few Nokias) however even for phones like the iPhone which don't currently support it, Vodafone having DTM support in their network means that an active data session will NOT block incoming calls. Even though Vodafone use NOM 002, if you are downloading data on 2G and someone calls you the data session will be interrupted and the call will come through.

Try it and see :) At the time I last tested it (a few months ago) Vodafone was the only UK network to support DTM, and to allow calls and SMS to take priority over data, and I don't think the situation is any different now.

[Ben]

Well, that's good news :) Perhaps why I haven't really noticed the problem on Vodafone.

There's that 'other' O2 reason why calls go to voicemail, of course. Their HLR regularly forgets about subscribers, classing them as absent when really they're online. Texts get retried and calls go to voicemail until an event occurs where the handset logs back on to the network. It's an anomaly I've witnessed dozens of times. Oh O2, why must you be so awful.

[a_ukboy]

Afaik Vodafone is the only network where calls don't go to voicemail when using 2g data! It definatly does with Orange, which is why I left them, too many calls straight to voicemail, been with Voda 14 months now and never have the problem anymore