And you don't have to put them on hold to do it? Entering the characters doesn't make them wonder what that beeping sound is?
Nope. Connect the call. Activate speakerphone. Open the menu screen. Open SMS app. Compose, send, read, whatever. Reopen phone app that was running in background. Or don't since your call is still connected and you can hear and speak. It would demonstrate poor work ethic, for instance, if one was reading TalkLeft in a browser window while participating in a conference call. Probably against some company policy. As such, I plausibly deny ever having done it.
If you have your keypad sound set it will make a noise. I turn mine off for a reason that may or may not exist in the above paragraph.