Of course, but if you call a Croatian number, are you likely to speak to someone in Croatian if the call centre is centralised? I suppose it is possible that they have enough Croatian, Hungarian and Polish speaking staff in one place, and they divert the call to the relevant staff based on which number you call. Obviously English speaking staff isn't a problem, so there isn't really a need for a UK based call centre.
Speaking English and understanding English are completely different things though.
Often these call centres have no clue what's going on either and work off standard replies and using just their website to advise passengers with no additional internal tools.
It is just the UK market has NX which offers more services and uses contractors who do it NX's way, unlike flixbus which does not tell anyone what coaches to use or what ticket kit they need.
Flixbus does kind of dictate to operators. I believe new coaches are to their spec.
Tickets they have a driver app of some kind which all drivers have to be trained on. They only don't specify ticket sales because they don't do them on the bus, NatEx does.
Cancelled services
Flixbus can't replace as it is a franchise so they have to hire in their own replacement but they don't as that costs them.
Flixbus could but it would cost them. Depends how much they value their reputation. If they cared for passengers, they would just not pass the revenue onto the operator and instead fund the replacement bus. But as Flixbus don't care about customers, they wouldn't do such a thing.
the Terms of travel
flixbus terms are what EU law says they can get away with. So they can just cancel and refund fares and only offer refunds after 2hrs delay.
Customer care
All the customer care is not done by flixbus but a 3rd party and station agents. Ticket agents can't change tickets but customers have to use self-service for any ticket changes.
Foreign customer care is useless. You could tell them anything and they'll just reply 'sorry to hear about this experience, it has been noted' or something along those lines. They don't even read the email.
Flixbus makes money by having as few staff as possible but on the flip side, more staff and better customer service would get more people on buses. Especially when I've emailed in to report problems on their website like stops not on the map so people don't know the routes exist and they just don't care.
For me, the other issue Flixbus faces is advertising. On street advertising is shocking with most stops having no information, or if they do, certainly not useful information.
Birmingham, what would probably be considered a core stop, when it was Dudley Street, there was nothing more than a bus stop flag with Flixbus on. No information on the service, website, contact details, nothing really for passengers. It's awful.
The only promotion that Flixbus has is online and that seems to be generic.