I don't watch Eastenders but . . .
Call centres work on behalf of other businesses, rather than running those businesses themselves.
So if, say, you phone up an insurance company called ABC Insurance, your call might be taken on their behalf by XYZ Call Centre Service (but with their staff saying "ABC Insurance" when they answer your calls). All those XYZ call centre staff need is an office, some phones, a scripts to work from when answering calls and a way of referring tricky calls on to the client. (e.g. by promising to get the client to phone the caller back the following day). So Nish & Suki's call centre might simply be an office that handles basic incoming calls for a pest control company.
Other call centres only handle outgoing calls, which are almost always unsolicited by their recipients. (Indeed, the majority of call centres probably handle outgoing, rather than incoming, calls). So, for example, Nish & Suki might have a contract from a pest control company, requiring their call centre operatives to cold call many thousands of businesses in their area, trying to sell the company's services. Once again, all that's needed is an office, some phones and a script to work on.
Some call centres don't even need an office these days, as they operate 'virtually', with all of their staff actually working from home. Both the set-up and ongoing costs are therefore kept to an absolute minimum.