It's a VOIP radio that uses cellular data.

You'll still get charged for roaming data.

There's still a "base station" somewhere, just not under your control. The Cignus models will be calling home to a Cignus server.

IMO just call from your favorite messaging service (Whatsapp, Viber, Line, Discord etc).

Or if you really want PTT, just use the Zello app. If it's for use on an overseas trip there's a free 14-day trial. Monthly cost of $8/month and activate only when you need it will probably be cheaper than buying separate Cignus units for everyone in your group then leave the device unused for months.