Something very small like this? www.kurioworld.com/k/uk/parents/products/pocket, though I'd rather it be something which can also run a VPN and use LTE.

Would this be logical for using for secure calls?

Depends on what you want. Honestly that tablet, while it fits what you want it for, is running a really outdated version(4.2 was released in 2012/13 I believe) of android that isnt very secure, even without having any google apps on it. if youre basically wanting a phone for just using data, i'd instead recommend getting an older nexus phone like a Nexus 5 and putting a custom rom (Android 5.1.1 or above) on it. Lollipop added lots of improved security features and better/easier encryption, so if it has that or even 6(marshmallow, which is easy to get on a nexus device) you should be in good shape.

Then you can decide whether or not you want GAPPS(all the google stuff, including the Play Store) or just want to use an open source app repository like F-Droid, which has lots of secure chat apps like Telegram(only if you use Secret Chats, i think you can send voice messages as well in secret chats but im not 100% positive on that), or an encrypted texting app like Silence. If you want encrypted voice, you'll need GAPPS in order to download Signal, which is basically Silence with voice and uses data instead of SMS for encrypted messaging. All those are predicated on the person you want to communicate with using those apps as well.

hope that answered your question, dont be afraid of rooting and adding roms. it requires some reading up on the subject, but its not hard once you get the jist of it.

Your first suggestion about looking for a Nexus device is valid if nothing else for general device security and device level encryption as you mentioned. If he's using anything prior to Android 5, he'd need to use a custom secure ROM that is likely not updated or supported much if any anymore.

I don't believe he'd need to other apps though if he's strictly using Silent Circle as it handles secure voice, data and chat within the app itself.

That's true, and i got silent circle mixed up with another dev who puts their stuff on fdroid. Looks like silent phone is not,not sure if there's any voice app that would on there besides telegram, which would be more like a walkie talkie.

Right now, I'm thinking about a hot spot and a VPN - will ask about VPNs on another thread.

I realized that I won't be taking any calls I need secure in public places, and I probably shouldn't take any while driving - so if I have a rapid boot lap top, and a hotspot w a VPN - it would serve my needs, along w a flip phone