By default all calls made to or from the iPhone will play through the ear speaker at the top of the device. However, if you wish you can set the iPhone to default to using Speakerphone mode when making or receiving a call.
You might set your iPhone to default to speakerphone mode if you are in a situation where it is uncomfortable or difficult to hold the handset for an extended period of time. It can also be useful for troubleshooting iPhone issues.
Tip: – Sometime the iPhone can become stuck in headphone mode, usually because dirt becomes lodged in the phone jack. However, if blowing into the phone jack port does not resolve the issue, you can try changing the “Call Audio Routing” until you can get a q-tip to clean out the phone jack port.