Some of the current ROM's won't allow PhonePad to load because of the number of processes being used at startup- like the Orange homescreen for instance. Even using SIPChange may not make any difference. I found that in it's default state, the M700 wouldn't open PhonePad even with SIPChange, but after disabling all the Orange stuff, PP worked after a soft reset, though not always!
ROM's which have fewer startup processes, like the new HTC ROM will allow PhonePad to work, without any tweaking.
So, the steps should be: Download & install the appropriate PhonePad software (the International Version seems most reliable); This is an HTC Copyright application, so you may be contravening licensing terms by installing it on a non-HTC device. Download and run SIPChange according to the instructions. Do a soft reset. Open an application to see the SIP Icon (by default a keyboard) and tap the arrow to the right- from the menu select PhonePad, which should then open. If it doesn't, you may need to consider a newer ROM, but again, bear in mind that this may have implications for your warranty