There are several applications out there (e.g. TAKEphONE, Datebook5, Agendus...) that take over the PHONE button.

If/when you remove any of those applications, you may encounter a situation where that button is left "hanging" - since the application assigned to it no longer exists, it behaves wrongly.

Normally, you could get over it by going to your system "Prefs" application, to the "Buttons" tab, and re-assigning the built-in "phone" application to that button (or pressing the "Default" button). To make things simpler - just install and run BtnFix, and press the "Go" button - this will fix the situation if needed.

Note: on the Treo 650 the "Prefs/Buttons" solution does not even work - it does not allow any change of that button assignment, so BtnFix is the only answer...