When all they had was WoW really that needed any sort of GM/CS support that was true. Diablo 2 and Starcraft had no real needs. WoW, SC2, D3, Hearthstone, HotS, OW, etc - they now have a MUCH larger total player base who uses Bnet accounts, the website, shop, etc. Billing and account issues make up a lot of it. For WoW they made a lot of the issues into Self Help tools so we now have the GM tools to do things like restore an item, un-delete a character, etc. With the bulk of it being account/billing issues it makes sense they go to one team who are cross trained.
Yeah, when land lines were still a thing and the other games had not released yet. It got to the point that it took hours of calling back to get through the busy signal. Then you sat in queue for 2 hours before your turn came up. All that time, wasting phone minutes if you were on a cell phone and crossing fingers your cell did not disconnect and you had to start over. Worse, it was supposed to only be for Billing and Account access issues, but people called for in-game stuff, GM jokes, to complain about the game, to try to appeal actions, etc. So a lot of the phone calls were not even things the phone support was supposed to handle.
They went to Phone Callback and Live Chat for Billing, Account access, and some Tech Support issues now. When available. Pandemic made a mess of that too.
A lot of time people are unaware of just what the GMs are allowed to help with and what they are not.
This article does a good job going over it all.