There's a theory about in the industry that customer service costs to much for too little reward.
Basically, these days you can provide excellent customer service, but that doesn't convert to loyalty. All customers are interested in is the cheapest price or best deal, lowest rates..and as consumers, we can't have our penny And our bun.
No-one wants to 'pay' for customer service,
Additionally, a lot of service providers are moving all of their support online, to extensive FAQ's and live chat options, to cater for a 24/7 customer base where you can provide links to websites, documents etc.
Anyone who provides a support service will tell you phone calls are the least efficient, least productive way to resolve a problem, and that way of communicating is often used by people who are , quite frankly, lazy and haven't tried to resolve the issue by themselves using the online support provided.
The argument that some people don't have access to the internet or are not technically good enough to do this doesn't really stack up, and if you are of a certain age or have a disability then most firms operate a scheme to register for where you can access phone support.
TL,DR - Phones are the least resourced support option from support providers, as it is the least efficient way of resolving an issue.