The capacity to dial *98 is dependent upon the dialplan of the ATA or IP phone you're using.
However, there are multiple ways to reach FPL's voicemail system and change your greeting.
You could dial your FPL phone number using your ATA or IP phone, using FPL.
You could dial a Freephoneline voicemail remote access phone number (useful from your smartphone) followed by your FPL account phone number (starting with 1) + #, followed by your voicemail password + #:
http://www.freephoneline.ca/vmAccessNumbers.
Or you could dial *98, provided you've setup your ATA or IP phone properly.
Here's an example using an Obihai ATA:
http://forums.redflagdeals.com/newegg-o ... #p28508685