Sounds like an ESP course, English for Specific Purposes. There are articles online that may be of help to you, but not much on Call Center English, so you'll have to develop your own core areas for the course, which should be easy to do as long as you know the kinds of situations and language your students will need to be able to function effectively in their job. Find out what the students need to know. That's your first step, and it's the most important step because the syllabus will write itself.
When I was asked to design an ESP course for Chinese teachers of English, the first thing I did was look up the acronym ESP and search what other teachers were doing or had done. Then I did a needs assessment: I asked my students, the teachers, about their job, their duties, the problems they faced, and what "they" wanted to know and learn:
Speaking; e.g., classroom management language, communicating with parents and colleagues;
Listening; e.g., strategies in aural testing;
Writing; e.g., lesson plans, language used in student assessments and evaluations.
Reading; e.g., interpreting language used in course textbooks; aural reading: intonation and stress.
ESP is specific in its focus; you don't teach the whole language, nor do you teach core skills. You meet the students' specfic needs.
Here are a few articles:
Teaching English for Specific Purposes
English Program for Indian Call Centers
English for specific purposes teacher: Job*description*and*activities
English for Specific Purposes: What does it mean