You're right about the whole phone number masking thing. It's pretty simple to implement these days, so any platform that doesn't do it is probably not all that concerned about looking after their customers. There's a taxi ordering startup here in Dublin where I live that has only 10 employees and they use phone number masking, if they can do it then there's no reason why Amazon can't.
The largest one is Cloudtail. Another is Appario.
Amazon just announced that they are parting ways on the Cloudtail joint venture.
https://www.reuters.com/business/amazon-end-relationship-wit...
https://www.business-standard.com/article/companies/amazon-i...
But still there are loads of sellers claiming to be an 'Amazon Brand'[1] on their listing. If Amazon does have something to do with them, Then again it's a violation of the regulations.
Then, What about 'Amazon Basics' products[2]? I'm sure Amazon manufactures them.
The whole thing is messy and keeps changing according to the 'Mega Billionaires' fight to hold their position, But if this results in temporary consumer choice until one of them wins and we have to pledge our souls then it's fine with me.
How much of it has to do with GDPR?
It could be argued that the taxi driver needs a customer's phone number if there is a problem, so therefore it wouldn't be an issue under the GDPR. The app developers probably only mask numbers as a courtesy, and not necessarily as a legal requirement.