IPhone 3GS. Lost the telephone icon from the tray. Cannot dial phone numbers. What happened? How can it back?

Go to your first screen and type "phone" into spotlight search. If it shows up, scroll through your screens and find the icon. To get it back to the tray, hold down any icon. When the icons start to shimmy, move the phone icon down to the tray. Press your home button to stop the shimmying.

If the icon doesn't show up in the search, backup your phone on iTunes and then reset to factory settings and backup from iTunes.

You've probably accidentally dropped it somewhere else in your iPhone on "jiggly mode." (If you touch and keep your finger on one icon, the icons will start shaking. From here you can delete or move icons.)

First, check to see if the phone icon appears anywhere else in your menus (scroll through your pages one by one or use the search feature). When you find it, put the phone back in jiggly mode––touch and leave your finger on the icon. When the icon starts shaking, use your finger to drag it back to the tray.

If this doesn't work, try plugging in to your computer and re-syncing your iPhone. Worst case scenario, you can reset your iPhone to factory settings.

Your phone may be in "Airplane Mode", in which case, you'd have to check the settings and turn this mode off.

Finally, that's what I was looking for! Thanks op of this question.