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Speaking on the phone is one the hardest skills to learn. It does depend so much on the other person being patient with you. Further to Yvonne's answer, I would recommend always to have a pen and paper ready. When you receive a call make notes, write down names, numbers, dates etc and always repeat back to the caller to confirm. If you are making a phone call, try and write some phrases you might need beforehand. The main thing to do is not to panic and ask people to repeat and slow down if you need. If anybody gets frustrated with you on the phone, you can always offer to continue the conversation in your native language! This usually proves a point and calms them down!

Hi Yvonne, ask them to speak more slowly. If you still don't understand say, "Thanks but we are so busy now, can you send me an email with the details we have just discussed and I'll get back to you". That way you will have time to look at the email and work out what is needed.