Customer Service jobs can take many forms: working a retail register, providing hospitality services at a hotel, conducting sales in the field, making or receiving calls at a call centre or in an office, and any other job that requires interaction with customers.
Customer Service or sales is typically an unregulated field, however, there can be cases where regulations do exist (e.g. Insurance Sales requires a licence).
The most important requirement in Customer Service is communication skills. The ability to use language effectively, politely, and in a nuanced way to effect a positive reaction from customers is a skill which takes time to develop and it’s a skill which one cannot be certified for – you need to show what you can do.