Is that a remnant from the Serco-Abellio days? I suspect customer service was operated by Serco then, and possibly Arriva just decided to continue contracting them to avoid having to set up new facilities and train new staff.
Customer Service in the Serco-Abellio days was in fact provided in-house by a small team based in Leeds. They were mostly experienced station staff, and the job paid a noticeable premium over the front-line retail staff. One of the staff was a near neighbour of mine who had previously worked at Todmorden and Hebden Bridge before I joined Northern, so I got to hear a good deal about the job - on more than one occasion she suggested I apply for a vacaant post there.
When Arriva took over in April 2016, this office was summarily disbanded as Arriva had their own centre based in Sheffield. My neighbour had already taken voluntary severance in one of the earlier rounds offered by Northern, and the other members seemed to find their way into various station retail and admin posts. I assume this office has continued to serve Northern since Arriva's departure, if anyone could confirm?
I think the Sheffield office was probably outsourced rather than directly staffed by Arriva, but to be honest, its provenance didn't trouble us station staff. What
did trouble us was a tendency for them to make wrong decisions regarding fares disputes. If in any doubt, the old Leeds office would probably have rung the station to discuss the matter, and if they were going to overrule the station staff, would explain why that decison had been taken.
Stations used to fax requests for disabled assistance through to Customer Services in Leeds for input into the system, a fact that was completely overlooked in the 2016 franchise change. We were initially told to use the Northern public online application form, but that was completely uselss for our purposes. A means of emailing a scanned copy of the written form to Sheffield was soon established, which actually worked very well.
As
@ainsworth74 says, the Twitter responders are in-house staff attached to the Control function. However, these seem to be recruited "off the street" for their lively personalities, make of that what you will.