Have you checked to make sure it isn't a text or email coming through while you are on a call?
Some phones have the option of enabling the Connect Tone where it will beep/make a noise when your call connects. Some phones also have the option of Minute notifications, where it beeps for every minute you're on the call. Not sure if the iPhone4 has this, but it wouldn't hurt to check your settings.
If you are still having trouble with the beeping sounds during outbound calls, I recommend adjusting your sound alert settings. I have provided some instructions below:
o Keyboard Clicks
1. From the home screen, select Settings.
2. Select Sounds.
3. From Vibrate, drag the switch left or right to enable / disable.
Vibrate mode enabled
Vibrate mode disabled
4. From Ringer and Alerts, adjust volume as desired.
5. From Change with Buttons, drag the switch left or right to enable / disable external volume buttons.
External volume buttons adjust volume of ringer and alerts.
External volume buttons enabled
External volume buttons disabled
6. From Vibrate, drag the switch left or right to enable / disable vibrate sound.
Vibrate sound mode enabled
Vibrate sound mode disabled
7. Select Ringtone.
8. Select a ringtone.
9. Select Sounds.
10. Select Text Tone.
11. Select a text tone.
Select None to disable text tone.
12. Select Sounds.
13. Drag the switch left of right to enable / disable the following sound options.
o New Voicemail
o New Mail
o Sent Mail
o Calendar Alerts
o Lock Sounds
Why is my iphone beeping randomly during phone calls (when I make the call). When someone calls me it doesn't beep.
Also I have been losing calls after all that beeping.
Beeping during a phone call could mean any number of things, like an incoming text/pix/flix message, call waiting, minute minder, any sort of push notification (depends on the apps you have) or even loss of signal/connection
Being that you are losing calls after the beeping, I am thinking that the beeping is signifying that you are in a bad service area and you just lost your signal, though I do find it strange that it only beeps on outgoing calls