could be a pair of walkie talkies. If most of your communication is with your travel partner.
Can't comment on the SIM issue, but I am aware of cell restricted environments such as our cruise ship. It is incredibly expensive to use your cell phone on a ship, because THEY are the provider, and they get to charge you out the wazoo. Even for "local" ship calls.
We got 2 pairs of walkie talkies, high power, that work throughout the entire ship.
So if most of your communication is based on being in a hotel on different floors (lobby->room), pool->restaurant), or splitting up while walking the local market place, the WTs will work for you. Then if you ended up using the phone for an occasional "emergency", it wouldn't feel (and cost) so bad even if you went for the most expensive plan, and you'll be comfortable knowing it will work when you really need it.