Which phone number are you referring to, the one displayed under "About Phone" ? That is not user editable as far as I know.
As for Voice mail icon, what carrier and what voicemail app? Some carriers like Verizon and AT&T include a voicemail app with their locked phones, You can also use Google Voice for voicemail and I assume other Play Store apps.
Galaxy S21 Ultra 512g(VZW), Galaxy Book (original, VZW), Tab S4, Galaxy Watch3 (VZW)