None of these are unique to iPhone 4S on Verizon or Sprint, but they're something to keep in mind if you're thinking of switching to one of those networks now that they have iPhone 4S.

iPhone 4S is the first iPhone to have both Verizon and Sprint CDMA versions available alongside the AT&T GSM version. CDMA is famous for its rock solid reliability and call quality and now that iPhone 4S is a world phone, it can even roam on international GSM network. However, CDMA brings with it some downsides as well, including lack of simultaneous voice and data over 3G, subtle quirks to SMS, and greatly reduced conference call options.

There are some great advantages to CDMA. It tends to need less towers to cover greater areas so CDMA networks typically have better reception in more places. This also means they tend to sound better, more often for voice calls. If you use your iPhone as a phone -- who knew? -- and that's the most important thing to you, or if Verizon or Sprint provide the only reliable coverage in your area, than case closed.

If cost is a factor, well we've already put together that breakdown for you.

But if you want to know what you're getting into when it comes to the data and conference calling tradeoffs, there are several things to consider.

Note: We'll be using a lot of tech terms, so see our wireless networking guide for more on what all these acronyms mean.

Slower 3G data

iPhone 4S on AT&T and other GSM networks supports 14.4 Mbps HSPA+ download speeds. Depending on where you live, you may never get anywhere near that speed in the real world, but the potential is there.

By contrast, while CDMA-2000 give Verizon and Sprint great voice, their EV-DO rev A 3G data speeds are theoretically limited to about 3.1 Mbps.

No simultaneous voice and data

Verizon and Sprint's EV-DO Rev. A network does not support simultaneous voice and data the way HSPA and HSPA+ does for AT&T (and other GSM carriers).

That means if a call comes in while you're using 3G data -- surfing the web, Skyping, sharing your connection via personal hotspot -- you can either ignore the call and continue using 3G data, or answer the call and effectively put your 3G data connection on "pause". If you're on a call you won't receive email or push notifications and if you try to surf the web or download an app you'll be informed you're not connected to the 3G network. Once the call ends, 3G data reconnects and you can start using the internet again.

If you're on Wi-Fi as opposed to 3G data you can make calls and use data without a problem. It's only 3G data that cuts out during calls.

Verizon and Sprint have chosen not to roll out EV-DO Rev. B, which does support simultaneous data in favor of more quickly deploying a 4G LTE network (currently a hybrid CDMA-voice with LTE-data network, in the future a Voice over LTE network) and WiMax with LTE to follow, respectively. There are some phones that support Voice over Rev A (VoRA) aka SVDO which will allow for simultaneous voice and data on CDMA/EV-DO phones, but not the Verizon or Sprint iPhone. (See The Cell Phone Junky for more on the technology.)

SMS/Text splitting

SMS/Text messages are limited to 160 characters. On many phones, if a message exceeds 160 characters, the OS will be able to put it back together and still show it as a single message to both the sender and receiver.

On CDMA networks this doesnOn Verizon once an SMS hits 160 characters, any additional text is split off into a second message, after 320, a third message, etc. The same content is still delivered, it's just not presented as nicely. (In some cases the message parts might even appear out of order which is even more annoying.) Some users claim to seldom if ever experience this problem in the real world, others say it happens often enough to annoy them.

Limited conference calling

Verizon and Sprint's CDMA networks only supports "3 way calling" so you can only enter into a conference call with up to 2 other people (3 including yourself) at the same time. It doesn't matter if iPhone can handle more, CSMA networks and hence the Verizon and Sprint iPhone is limited to 3-way calling.

Likewise, handling conference calls is also more challenging on the CDMA iPhones since you can't take one party "private" or hang up on one caller while keeping the other active. You can only hang up on all of them at once.

Apple provides the following diagram in their knowledge base:

The network

So yes, overall there are a lot of limitations to the way CDMA handles voice. However, if Verizon or Sprint has great coverage in your area, you're on Wi-Fi when you want to talk and surf, split SMS/Text messages rarely happen or don't bother you, and agile conference calling isn't a must-have business feature for your iPhone, you may not care.

Otherwise it's a compromise. Decide what's most important to you and which carrier best provides it. If anything is a deal-breaker, then that makes your choice much simpler.

For more information and help check out our iPhone 4S Forum.