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/Lea...at/td-p/430933