The larger issue is that the prepend affects the redial of calls in the phones history list. The pretend as some people noted was a trunk access account or security code that prepends the number in the sip call request. I imagine the designers thought of the prepend as more of a way to insure the full dialing format of a shorter number.
Perhaps the cleaner way to solve the issue is to allow a prepend in the trunk settings. Either way it’s a limitation that should be addressed.