Is this a good idea for a backup sim card?

No luck.
And here's something very strange -- there's no way to check GV voicemail by calling your GV number from a different phone! When did this happen? And is it happening to just me?

I tried a bunch of different keys when the voicemail greeting was playing but the only thing GV lets me do is leave a voicemail. I tried *, #, *, ##, #, #, 0, 1, 0, and a bunch of other key combinations, but it won't let me enter my voicemail PIN.

(Unlike dst11, I'm calling my GV number using a smartphone. Not the smartphone that is tied to my GV number but a different one)

Hopefully someone can shed some light on this deepening mystery!

A long shot, but is it possible the VM greeting you are hearing is from your smart phone's carrier's VM rather than from GV VM? You may have already checked this by leaving a voicemail or heard the "GV subscriber" in the GV message.

"When someone calls your inbound Google Voice number, Google Voice subsequently and simultaneously forwards that call to all of your linked/forwarding destinations. It will wait about 25 seconds of ring time. Whichever one of the forwarding destinations answers first, wins the race and takes the call. When a forwarding destination number is busy, or unanswered, THAT phone number's carrier number is programmed to send the call to its voicemail system. That behavior is controlled by that number's carrier. It's a problem, because it creates an unwanted behavior - the calls end up on your carrier's voicemail instead of Google Voice's voicemail."

https://support.google.com/voice/thread/1808152?hl=en

I get GV's voicemail with the "google voice subscriber" message. And after that, no matter what key I press, I can't enter my voicemail PIN.

Is this happening to anyone else? How do you guys get into your GV voicemail when you call your GV number from a different phone? This is getting very perplexing.

is the "different one" tied to GV? when I first called GV, my mobile wasn't linked to GV, GV said so and nothing more happened. then I linked it and when I called everything went OK.

I know that calling out via the spoofed numbers requires that the phone be linked and, I believe, set as an active forwarding number.
At least linked might be required for the method of calling a landline/unknown-spoofed number-- but I don't know for certain.

It doesn't seem logical to limit voicemail retrieval to only linked numbers-- but I've never retrieved messages this way.

My 'backup' phone need is, and has been for AT&T based services.
Having run the gamut of what is available - I've sent along the obnoxious FU SIM to someone who needed it more, giving away the hated TF while it still has some of its quota remaining - and since Redpocket wants $100 for the plan that they didn't offer any better deals on this year - it is time for a Truphone SIM and to be done with the searching phase.
(I even tried the free Mint SIM, but it was just another time waster along the way for me.)

Time to order that SIM now & to proceed as I detailed here:
https://www.nthcircle.com/forum/other-services/69-truphone?start=30#43000

Has EZKonnect closed down? Their site doesn't load at all for me. I hope it's just a temporary glitch, and not a permanent shutdown.

Just loaded for me, but it's a bit slow when maneuvering about.