To launch TealPhone when you issue a command to create a new address item in DateBk4 or DateBk5, or when you tap on the GoTo/Edit button in the AddressBook details screen, you need to set the Creator ID to 'TlPh' (Capital T, Small L, Capital P, Small H) in the 'Addr CreatID' field. This item is located in the 'Split' tab of the main preference panel in DateBk5 in DateBk4 (tap on menu icon at top left).
To have TealPhone launch when you press the AddressBook button, you should first program the AddressBook button in the Palm OS to run Tealphone:
Tap on 'Prefs' icon in main application screen, select the 'Buttons' panel and select TealPhone from the popup list next to the AddressBook button.
Next, you can either have DateBk4/5 ignore the AddressBook button open up a split-screen display from within DateBk4/5 (in that case, the split screen is drawn by DateBk4/5 and is not under the control of TealPhone, but a 'Quick' double-press of the key will still launch TealPhone. This preference item is in the 'Views' tab of the main preference panel in DateBk5 (select '2' to enable DateBk5's use, or a hyphen (-) to disable it), or in the 'Preferences' dialog of the Split-screen menu in DateBk4