While many have been waiting a long time for the Verizon iPhone, the same CDMA network that gives Verizon its terrific coverage and reliability brings with it a host of other problems, including the famous lack of simultaneous voice and data and limited international roaming, but also some lesser known issues such as split SMS messages and greatly reduced conference calling options.
None of these are new to the Verizon iPhone but they are new to iPhone now that it's on Verizon, and new to users who haven't experienced CDMA before. We'll take a look at them after the break.
Verizon runs on CDMA2000 using EVDO Rev. A for 3G data (see our wireless networking guide for more on what those terms mean). EVDO Rev. A does not support simultaneous voice and data the way AT&T (and other GSM carriers) HSPA networks do. 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 that cuts out during calls.
Verizon has chosen not to roll out EVDO 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). They are rolling out Voice over Rev A (VoRA) aka SVDO which will allow for simultaneous voice and data on future CDMA/EVDO phones, but not the Verizon iPhone. (See The Cell Phone Junky for more on the technology.)
GSM/HSPA, the standard used by AT&T and T-Mobile in the US, is far closer to being an international standard than the CDMA/EVDO technology used by Verizon. While there are a few other CDMA/EVDO networks in North America and Asia, GSM/HSPA is supported throughout most of the rest of the world, including Europe. If you use a Verizon iPhone there are a limited number of countries where you'll be able to roam. Verizon does offer loaner GSM phones for international travelers but that's not as seamless a solution as GSM iPhones that can intrinsically roam around most of the world.
SMS/Text messages are limited to 160 characters. On AT&T and other GSM iPhones, if a message exceeds 160 characters it will still be shown as a single message to both the sender and receiver. On 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.)
Verizon's CDMA network 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, Verizon's network and hence the Verizon iPhone is limited to 3-way calling.
Likewise, 9to5Mac points out that handling conference calls is also more challenging on the Verizon iPhone since you can't take one party "private" or hang up on one caller while keeping the other active. (It hangs up on all callers.)
Apple provides the following diagram in their knowledge base:
So yes, overall there are a lot of limitations to the way CDMA handles voice. However, if Verizon has great coverage in your area, you're on Wi-Fi when you want to talk and surf, you rarely if ever travel internationally, split SMS/Text messages 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 Verizon iPhone Forum.