Honestly, the website is your best source for info.
Buy a device, install the app, activate the device, chose a subscription (default for initial activation is 3 months of the unlimited), then text without cell signal. It also works with wi-fi and cell signal.
I timed my initial activation to cover my last hunt of the year (mid-January) so I had coverage mid-OCT through mid-JAN. After that, I dropped my subscription down to "suspended", which allows you to keep the same Zoleo number and email, for $4/month. I'll activate mine again in October and run through mid-January. If I remember correctly, the unlimited is around $50/month.
There is a delay in messaging from person to person, but it isn't horrible. The default setting is that the device pulls and pushes messages every 12 minutes. Worst case delay between you and a hunting partner is 24 minutes. You can adjust the time between "pulls" but I haven't played with it.
I do not believe this delay pertains to the SOS feature but cannot confirm.
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