In Denmark we do not use foreign call centres, as no-one in them speaks Danish, but we do have the problems you describe.
Phone communication is becoming difficult, because of the mumblers and the age gap between us and the youngsters answering the phone.
I simply, the first time I fail to hear clearly what was said, say politely, excuse me, could you speak a little slower and more distinctly ?I am having trouble hearing you clearly.
Usually, the person on the line obliges.
If I fail to understand the technical jargon, I likewise say so, apologising (it is a marvellous way to get a youngster to help, I find) for my lack of technilogical expertise.
Written communication has completely different problems these days.
Irrespective of whether you write a letter or an e-mail, people now tend to answer the first question you ask, and not bother about any subsequent queries.
Or they answer the question they thought you meant, not actually what you did ask them.
Not that this is a new thing.
In the dim and distant past when we were still all using electric typewriters (remember them?) I took mine to the repair shop four seperate times because in the middle of writing on it, the entire carriage would suddenly start juddering and shaking, so it was quite impossible to type.
Nothing I did, stopped this - you just had to wait for the typewriter to stop of its own accord.
I described the problem carefully, because naturally the typewriter wrote perfectly in the repair place. When I collected it, they insisted they had rectified the fault, but no. On the last occasion, I insisted on turning it on in the shop, and lo and behold! the carriage shook and juddered to beat the band.
"Oh, " said the repairman, "that is something completely different to what I thought the problem was." My reply was that I had minutely described this phenomenon to him no less than three times. Finally, he fixed the typewriter and it never repeated the fault.
Medical professionals, even those on geriatric wards, seem to be largely unaware that most of us experience some loss of hearing as we age. Why this fact is no longer rammed down their throats I do not know, but I attribute it to there no longer being the kind of ward sisters who scared housemen and student nurses silly!