The Selectel site intrigues me, but does not give enough info. Its CDMA, so that's the Verizon network, right?
Yes, Selectel is a Verizon MVNO. You put the Verizon restriction in place, as such, I wouldn't suggest otherwise. That said, CDMA is not exclusively a Verizon network technology. Both Sprint and US Cellular use CDMA as well.
No, switching to Selectel wouldn't solve your iPhone tethering issue specifically, just the terms of service issue with your current Verizon MVNO. You'd still need to jailbreak to make the tethering happen. Also, I missed the announcement yesterday that Page Plus has started doing the whole "unlimited" throttled data thing with some of their plans, no wonder they changed the terms of service to technically make tethering a violation.
Page Plus officially supports activating iPhone 4 and 4S handsets now (as of about 6-9 months ago, IIRC), and I think the 5 is officially permitted now as well since the LTE support rollout in October, but I always recommend taking the Verizon iPhone 5 to a GSM MVNO instead of sticking with Verizon unless you just absolutely need Big Red's network coverage.
The only used device that you could currently activate for a data only account with Selectel is a MiFi 2200, as they don't do 4G LTE devices as of yet. Contact support to see if you can activate a used Verizon Novatel MiFi 2200 with a clean ESN. If they say yes, you might be able to pick one up off Ebay for around $35-40. Not a great savings over theirs, but it is a technical savings that could potentially be shot if you have to replace the battery. Just be sure to ensure you buy a unit with a clean ESN, and it HAS to be a Novatel MiFi 2200 and Verizon branded. Clean ESN, Verizon branded, Novatel MiFi 2200. Got it?
I highly recommend against trying to do VoIP over a mobile data connection for a multitude of reasons, but for the technically curious, most usable codecs on mobile data use about 7-10KB/second, or around 500KB/minute.