Q: How do I fix call waiting on my iPhone 4S?

Hi MDD82,

Welcome to the Support Communities!

The article below may be able to help you with this issue.

Click on the link to see more details and screenshots. 

iPhone: Understanding phone features

http://support.apple.com/kb/HT4515?viewlocale=en_US

Page 45 of the iPhone User Guide explains what do you when you have an incoming call:

manuals.info.apple.com/en_US/iphone_user_guide.pdf

Cheers,

Judy