Iridium currently offer a very basic entry-level Emergency Plan for $25 a month, which suggests that you will not get any emergency facilities without an active plan.
You don't say what your usage will be, but before you make your choice you should research the pros and cons of your three main options.
The PLB is, currently, one-way only as you say. But compared to your other options it generates much the strongest signal, has by far the most reliable satellite reception, and will work under heavy tree cover and in mountainous areas, deep valleys and even gorges. Recent models will automatically communicate your GPS coordinates and broadcast a homing signal to rescuers. It also has a sealed, long-life battery. And it connects directly to the COSPAS-SARSAT global rescue dispatch service. It is much the most reliable way to call in the rescue, and does not require a costly ongoing airtime plan to work.
A quality satellite phone offers by far the best communication experience, with the choice between text and voice. But compared to the PLB, it is much more expensive to purchase and run, and it has far less reliable reception. Nor does it automatically communicate your position to rescuers. You also have to be careful to keep its battery topped up, and the regular charging/discharging cycle means that its battery life will be more limited.
These are a half-way house between the phones and the PLBs. The cheap-and-cheerful Spot offers one-way text communication, and the more upmarket InReach offers 2-way texting. But compared to a PLB, reception is much less reliable, you have ongoing service charges, and you have to keep the battery topped up.
Choosing a service
So the best solution will depend on your priorities and budget.
On a lightweight solo walk where I could only take one device it would be a PLB, as my priority would be reliable rescue.
If your priority is keeping friends and family informed on a modest budget, you'd probably prefer a messenger. But be aware that rescue will be significantly less reliable.
With a larger party in a serious and remote location, the ideal would be to carry both a PLB and a satellite phone. That way you can reliably call in the rescue in-extremis, but also have a good chance of being able to communicate with friends and family, with medical and rescue services, get access to weather reports, and organise logistics. Given the overall cost of such a trip, the relatively modest additional costs of this arrangement would seem like a good investment. People never think it can happen to them - but believe me - it can.