Do refer this and see if it works for your existing number:
https://faq.whatsapp.com/general/im-traveling-and-i-cant-get-my-whatsapp-code/?lang=en
You can access the voicemail of your number at https://vp.virtualglobalphone.com/VUP login with your SIP credential.
If you want to register a new number on the same APP than delete the respective account and then uninstall the whatsapp. Install the app all again and follow the above steps.
Please note our support doesn’t cover whastapp nor we claim our service is whatsapp enabled. You have to check with whatsapp support team ; what type of numbers they will allow to register.