Just look them as your friends and chat with them natural,that's ok.If you make friends with them in your honesty,they will treat honesty in return.

Google IT!

I encourage you taking International business course or you can study it by your self, you deal with customers from different countries so this course will enable you to understand different cultures, languages, norms,customs, religions, contact distance and power distance and other aspects. General advices for whom contact customers by phone. Your aim must be conduct your contact with HIGH QUALITY . in this situation you have to put your personal touch during the call because you have not any tangible product by which you can add some features and convincing the customer, you have use the customer name at least three times during the call, you have to distinguish between hearing and listening, you have to feel with customers need especially in complaint situation (emotions is very important), you have to be honest with the customer and don't promise him/her something you can't do, its very important to gain customer trust,use simple word and avoid jargons, finally allow customer talk and talk and don't interrupt him/her but don't allow him/her to lead the call.

Just tell them what you want to express exactly. Make it efficiently.