We previously noted that the iPhone may miss calls while sleeping (locked). Some users are now reporting what seems to be a significant issue where their iPhones cannot receive incoming calls while transferring EDGE data. Here, in a nutshell, is what appears to be happening:

The iPhone cannot simultaneously use EDGE and voice services. That is, if you are on a call, you cannot concurrently access EDGE-data functions. In addition, as documented in Knowledge Base article #305711:

"While iPhone is actively transferring data over EDGEâ??downloading a webpage, for exampleâ??you may not be able to receive calls. Incoming calls may go to voicemail."

The "may not be able to receive calls" portion stems from the fact that there are two types of EDGE network types, NOM1 and NOM2. When your iPhone is connected to a NOM1 network, the data transmission will generally be interrupted, and the incoming call allowed to come through. When your iPhone is connected to a NOM2 network, however, the EDGE data transfer process cannot be interrupted, and the call will generally go to voicemail.

Fortunately, there is a way to check whether you are connected to a NOM1 or NOM2 network. First, put your iPhone in field test mode by accessing the Phone application, tapping Keypad, then entering *3001#12345#* and pressing Call.

Tap GPRS Information and look at the entry next to nom. It will be either 1 or 2. If you see a 1, you'll likely be able to receive a call while transferring data. If you see a 2, you likely won't.

Unfortunately, it appears that NOM2 is much more prevalent on AT&T's data network. In fact, we've yet to see our iPhone connect to a NOM1 network. As such, we can't even state with certainty that the iPhone supports NOM1 at all.

Here's a more detailed description contained in a patent application for "Limiting services based on location."

"GPRS network 130 can be designed to operate in three network operation modes (NOM1, NOM2 and NOM3). A network operation modes of a GPRS network is indicated by a parameter in system information messages transmitted within a cell. The system information messages dictates a MS where to listen for paging messages and how signal towards the network. The network operation mode represents the capabilities of the GPRS network. In a NOM1 network, a MS can receive pages from a circuit switched domain (voice call) when engaged in a data call. The MS can suspend the data call or take both simultaneously, depending on the ability of the MS, In a NOM2 network, a MS may not received pages from a circuit switched domain when engaged in a data call, since the MS is receiving data and is not listening to a paging channel In a NOM3 network, a MS can monitor pages for a circuit switched network while received data and vise versa. "

source