OP has qualified his question to be specific for mountaineering, and has clarified to mean 'best practice' instead of 'required by law or guild'
In mountains many mishaps are close to instant, or are weather related. A communication device may just mean that people get the unpleasant news sooner.
If something happens, being able to call for help is not as useful as you might think.
The flight ceiling for most helicopters is about 12,000 feet. Many pilots are not comfortable at landing with low power reserves except in perfect weather. They also need a reasonably flat spot to land.
A surface based rescue party has to deal with the conditions that the original party did.
It's really hard to get lost on a mountain. Oh, you might not know for sure which ridge you are on in a white out.
Mountaineering folks may make mistakes, but generally they will have had enough experience from other outdoor activities that getting lost is not one of them.
So the first part of this answer is, "It doesn't help much"
That said, while getting lost may not be likely, mishaps happen. So some form of emergency communication is prudent, and for anything more rigorous that a walk up, recommended.
And while a mishap at 14,000 feet may require being dealt with using only your own resources, you very likely have spend considerable time on the approach, where rescue is actually possible.
In my original answer, below, I mentioned costs. I'm not up to date on these, but I suspect that they are still in line with current high end cell phone costs, with significant plan costs.
So the second part of this answer is, "They are expensive"
The third part: What are other alternatives?
A: PLB This a location beacon that has a test button, and an auto/on/off button. The test button verifies it's working. In Auto it turns on automatically if it's hit hard enough. You crashed your plane. In On, it beeps. Off, I leave as an exercise for the student.
B: SPOT This device allows a "I'm ok, checking in, and perhaps with the latest version, one of a small number of pre-canned messages.
C: InReach. This device allows what amounts to text messages. The device has built in GPS, and includes coordinates with each message. You can also set it up to send an "I'm at x,y" breadcrumb at intervals. The low end service is about 30 bucks a month, and you can put your plan on standby for $3/month.
I went with this option as being the best tradeoff between expense and flexibility.
When I worked for a school that did lots of outdoor stuff, we did carry sat phones. At the time, they cost several thousand dollars, rented for $200/month (and a month was the minimum period even if you only needed it for a weekend.)
In order to be familiar with their limitations we phoned home daily. Air time was $2.50 per minute.
In general we found that we were successful about 85% of the time.
You needed a clear view of a reasonable amount of sky. Vegetation didn't seem to matter much. Mountains, especailly ones to the south did.
Dense fog seemed to interfere with the signal.
The early ones were very fragile. The wrong knock would turn them into a paperweight.
Now suppose you have one, and you have gotten lost. That you are lost means that you probably don't have a map, or don't know how to read it. And you don't have a working GPS. So with your phone, what do you say to the person at the other end?
It's been years since I worked at that school. Doubtless by now they have a GPS built in.
One of the reasons I go into the wilderness is to get away from the Nanny State.
The last thing I want is a bunch of regulations saying what I have to take into the wilderness. I already have given up on Canada's beautiful national parks because they are so over regulated.
I do carry an InReach communicator. Essentially I can send text messages from the wilderness to tell my spouse that the dog and I are still alive. But I do that by choice, not because some bureaucrat tells me to.