I do not think that you can use your phone as a speaker phone even though it has a speaker and a mic. I think that the speaker has to be set up as a microphone in order for it to work. I think that it is the way that the sound is routed.
Do you mean use it as an external microphone? I am not sure if this is possible, and it probably wouldn't sound all that great. The microphone in a cell phone is not very high quality. Your best bet would be to by a small external microphone.
Yes you can use your phone as a microphone, depending on what type of phone you have. For the iphone there is an app called megaphone that will convert your cell phone into a
microphone. If your phone has a voice record feature, there is a demo of how to use it as a mike on youtube
The quality and range of the microphone in your phone would make it less than desirable as a regular microphone. I know of no phone that would allow you to use it as a microphone. Many full feature cell phones do come with voice recording software.