Customer complaint

Answer:
Service complaints are, basically, your customer's feedback on service (quality, performance, etc.) or your IT Service Management (ITSM) organization. Therefore, they should be seen from positive side. Customer complain can come in several ways. here are few examples:
1. Through Business Relationship Management (BRM) - BRM is in (business) contact with customer and should be advocate of complaints received by customer
2. Through Service Level Agreements (SLM) - they continually communicate with customer (on operational level) and can also be a channel how customer can communicate their complains
3. Through customer complaint form - use e.g. Service Request for user to submit complaint. Or publish document template on the intranet which users can use to submit complaints.

And one more thing - once you receive complain, it's crucial that something is done and that customer knows that.

This document can help you with more details regarding customer complain"ITIL Customer satisfaction – Design driven by outcomes" https://advisera.com/20000academy/blog/2014/07/08/itil-customer-satisfaction-design-driven-outcomes/