In the HTC messaging app, under settings will be "Connection Settings" which will show the MMSC, but not allow you to change it. I have never gotten MMS to work using AOSP/stock Android messaging.
By entering basic provisioning (DATA/3282), EPST should
let you edit all settings. Let me know if the dialer doesn't open EPST. When EPST opens and you enter your MSL for edit mode, the MMSC is listed under "Advanced". Once you change the MMSC and go to Menu>Commit Modifications, it will tell you "No changes made" unless you change another settings (I just change the SCI to something different) I was able to edit it on the Fresh ROM I posted earlier, and my carrier uses Verizon's method for messaging.
For the voicemail issue, refer to this post
. Voicemail is usually the only issue I have flashing Sprint ROMs.