Hello My Fellow Sarsaparilla Sekai Ichis,

It doesn’t take long for those iPHone 3GS Apple seeds who make and receive calls, using only the handset, to realize that the new position-based auto-engage speakerphone feature is more nuisance than nuance. 

Until now, the only way to insure in-call remote party privacy on the iPhone 3GS was to use either a wired headset or a wireless Bluetooth headset during a call.

However, there is a way to quickly and easily take control of the iPhone 3GS Speakerphone, during a call, that does not require you to interact with the touch-screen. 

Here is how it works: 

1.
Understand that, during a call, the key hardware component that enables iPhone to detect that it is no longer pressed against your ear (automatically engaging the Speakerphone Feature) is the device’s proximity sensor.

2.
The iPhone 3GS proximity sensor is located near the upper left corner of the unit just above the display.  The sensor area resembles a rectangle. To locate the proximity sensor area:
A.
Remove iPhone from any case or cradle.

B.
Lay the unit down on a flat surface so that its Display Screen is visible. 

C.
Make sure the unit is positioned lengthwise (portrait).  That is, the [Home] Button should be just below the Display Screen.

D.
Gently touch the [Home] Button with your index finger and slowly slide up towards the top of the unit.  As you do so, your finger will first pass across the Display Screen and then you will feel/see a thin lateral slit.  This is the speaker from where you  hear the remote party during a call when the Speakerphone feature is not active.

Generally speaking, the area of the device located between the speaker and the left side of the phone, houses the proximity sensor which, in direct light, can be seen with the naked eye.

3.
During a call, do one of the following:
Cover the proximity sensor area with your thumb (for left-handed Apple seeds like me) in order to prevent the Speakerphone from engaging regardless of iPhone physical orientation.  Please note that when the speakerphone is inactive, the iPhone 3GS Display is temporarily disabled.

If during a call, you forget to cover the proximity sensor, allowing auto Speakerphone to become active, simply cover this sensor to immediately deactivate the speakerphone and return the call back to the ear-speaker just above the Display Screen.

And that, My Dear Apple Blossoms, is all there is to it. 

Enjoy,

Mark