[Verizon Galaxy S 5 Q&A, Help & Troubleshooting] Tech Critic:
I haven't looked into this in over a year, but I was under the impression that Verizon operates on an IMEI white list, i.e. ONLY Verizon branded phones are allowed on the network. When a user attempts to activate a phone on the network, it's IMEI is checked against the list of IMEIs for every phone Verizon has sold. If it's IMEI is not on that list because it's a non-carrier branded phone or it's another carrier's version of the phone it will not be allowed on the network regardless of hardware compatibility.

From what I recall, this was also the reason that Nexus 7 LTE buyers were screwed by Verizon. Verizon was advertised as supporting the tablet because legally they had to due to the open access provision they agreed to when they purchased their LTE spectrum. Up to that point Verizon had operated with a white list as I described above. They were able to get away with this despite the open access provision because they run voice and text over the older 2G and 3G networks on which they have not agreed to open access. When the tablet came out, for months Verizon had no system in place to activate it because the IMEIs were not on the white list. Users were able to successfully connect to the network with a SIM activated in another tablet proving that it was not a hardware issue.

Has this changed in the last year? If so that great news!

---------- Post added at 12:25 PM ---------- Previous post was at 11:58 AM ----------

I just Googled it, and based on information from November when the Nexus 6 came out, Verizon still operates on a white list, but it's only for activating NEW LINES. Verizon will not allow you to activate a new line with a non-Verizon branded phone, however, once the SIM has been activated, if it is placed in a non-white listed phone, that phone will be allowed on the network.

Some people suggest that you can take the IMEI of a floor model at the Verizon store and enter that when you order a SIM CARD, and that will allow for activation. I cannot personally verify this as I'm not a Verizon customer, and I'm not sure if the SIM activation process requires an actual phone with a white listed IMEI.

---------- Post added at 12:51 PM ---------- Previous post was at 12:25 PM ----------

Also OP, have you verified that the handsets with SMS problems support all of Verizon's 2G and 3G network frequencies? From what I recall, Verizon still runs voice and SMS over its legacy 2G/3G networks. I'm not sure if they've implemented Volte yet, but it's likely that older handsets and even some newer ones don't support it, especially if they don't have a Verizon ROM. It's possible that a handset could support only Verizon's LTE frequencies or support everything except it's 3G frequencies for example. If it supports only the LTE frequencies, data could work while voice and SMS don't. If voice and SMS are run over different frequencies it's possible that only one of the two will work.

Information on supported frequencies can be tough to find for phones sold in the US. The information is readily available for the same phones sold anywhere else in the world. The most plausible explanation is that this is because the US carriers insist upon it. They've been forced to start unlocking phones that in the past they've been able to permanently lock to their own networks for domestic use. To combat the emergence of a free marketplace where customers are free to move to a carrier offering a better deal, the carriers have started specifying that their carrier branded versions of the phone be shipped with firmware (possibly also hardware variations) that does not support the frequencies of other US carriers. That way, the phone is "unlocked," but it still won't work on other US networks, so they've managed to sidestep the legislation and continue screwing over consumers.

If the supported frequencies aren't listed on the manufacturer website, you can find them in FCC testing documents.

If customers do have a problem where voice or SMS doesn't work because the frequency isn't supported, they could use a Google voice number for calls and SMS over the data network using the Hangouts app. It works great for me. Your employer definitely would not approve of this suggestion though, since it compromises the way they've artificially limited their technology to force consumers to pay more. All calls and texts to/from a Google voice number are totally free when you're on a Wifi network, and the number is device agnostic, so there's no barrier at all to switching phones or carriers. Verizon wants to use your phone number to make switching as inconvenient as possible, so they don't want you using a cloud based phone number.