There’s only one Cellcom in the US. There’s a CellCom Israel, but this is pretty clearly a US story.

“The iPhone 5 is designed to allow customers to make voice calls on the Verizon Wireless network and surf the Web on Wi-Fi,” she said in an e-mail. “It was an Apple decision.”
That means when AT&T customers place a phone call and use data on the iPhone 5, both functions will roll back to AT&T’s older network, which can handle them simultaneously. When you place a phone call while using data in an app with a Verizon or Sprint iPhone 5, it will roll back to their older CDMA networks, which are not capable of simultaneously doing calls and data. And that’s why the iPhone 5 on Verizon and Sprint, despite being a 4G LTE device, will still not do both at the same time.

An Apple spokeswoman, Natalie Kerris, put it this way: “iPhone 5 supports simultaneous voice and data on GSM-based 3G and LTE networks. It is not yet possible to do simultaneous voice and data on networks that use CDMA for voice and LTE for data in a single radio design.”
Then why didn’t Apple add another antenna? Its phone already has two antennas in an effort to improve reception, and it would have had to add a third antenna just for Verizon and Sprint phones to give them simultaneous data and calls, Mr. Shimpi explained. Leaving that third antenna out allows Apple to simplify the process of manufacturing the iPhone for multiple carriers. Plus, in the next two years, 4G LTE technology is supposed to evolve to support voice calls, which would render another antenna unnecessary.
So why does Verizon’s Samsung Galaxy S III, a 4G LTE phone, juggle calls and data? Samsung added an extra antenna so that it pulls data from the 4G LTE network at the same time that it’s using another antenna to do voice, said Anand Shimpi, editor in chief of AnandTech.