I joined just to reply to this thread.
In my area, only Page Plus actually uses Verizon towers (Blue Ridge mountains foothills).I live in an urban area down a street that blocks all towers other than Verizon. Straight talk cannot get a decent signal at my home, although Page Plus can.I am uncertain how Straight Talk would be affected in various areas of the country, but it is my understanding that in Western NC and SC, only Page Plus will use Verizon towers.
Verizon recently made a change, effective January 15, 2015 that "unknown" or flashed phones( not approved on their network) can not be used by Page Plus. My family has been using four flashed "moto G" phones on 3 G ,with Page Plus for about a year, 3G has been fine for our needs, we don't stream much of anything..
When I ported a number out (my business phone) I tried to transfer the same phone that had been working for a year on page plus to another number currently on Page Plus. Page Plus would not transfer the phone, even when I told them if they did not I would port the number out. I was told that Verizon implemented a policy Jan 15 not allowing flashed phones on their network.
My point here, is that if Verizon has recently implemented this with Page Plus, it is quite possible they could prohibit flashed phones from other carriers (on new accounts) sharing their towers in other areas of the country.
This is a recent development ( and we still have three flashed phones working on Page Plus) You may or may not find info in this thread may be out of date ( as far as implementing _new service_)
Also, talked with the Page Plus local dealer (who had flashed the phones) and he said there is no workaround. Basically, Page Plus looks up the MEID in their system and it is approved or not.
I am aware that the OP is concerned about Straight Talk. However, if straight talk shares towers with Verizon in other areas of the country it may be that this is a concern for new users. Old users, their phones should continue working. If that phone goes out of service, you may not be able to replace the phone in the system - depending on what phone it is.
Hope this helps, at least as far as use of towers, and flashed phones with prepaid carriers on Verizon.