Relatively untraceable communications with ground-based collaborators
If there is a conspiracy involving collaborators on the ground, it has already been pointed out that satellite phone contact would probably work from inside the plane, especially near a window. A small hand-held VHF set would be problematic and give very restricted reliable range of miles or tens of miles unless you were lucky.
Whilst the use of a satellite phone would leave you open to eventually being tracked / triangulated that would almost certainly take days to occur as the satellite operators would have no particular reason to suspect that any given call was even remotely related to the missing aircraft.
From the flight deck, as may well be the case, HF radio would probably be your best bet. Picking a suitable channel, you are highly unlikely to be overheard and HF is notoriously hard to triangulate unless significant assets are coordinated and expecting your transmissions. This would give a potential reliable range of one or two thousand miles, coupled maybe with a switch to VHF when in closer range. It's easy to overestimate the SIGINT resources that are out there and kid yourself that 'everything is monitored' but it's vastly more likely that that is simply not true. And even if it were true, your goals have probably been accomplished long, long before anyone realises that a couple of random / unexpected transmissions were you and joins up the dots.
Simple portable HF stations are ridiculously easy to set up as anyone with a comms background (as I have myself) will know, ditto VHF. It can all be operated from the back of a truck and put together for a few hundred dollars.