I did a search and couldn't find anything about this.
I have the big button dialer installed and no sprint add-ons.
All other speed dial entries work fine (2-9)
although Voicemail is set up as a speed dial location (1) it never connects.
make sure that the speed dial for '1' is set to *86. I dont know what service you have but verizon's voice mail is *86. I also like to set it up to automatically plugin my pin so it automatically puts me on voice mail.