With regards to your first question, HulloMail requires conditional call forwarding to operate and not all carriers offer this so only the carriers listed when signing up for an account are available.
Please can you clarify what you mean in your second question.
I am using Verizon as my phone carrier. The Communication protocols you listed for this carrier are CDMA and TCDMA, neither of which are currently being used by Verizon. Their network uses GSM LTE (just called to check) Are you currently or will you in the future supporting this protoc0l? I dearly need to use your HulloMail app. Solves many if not all of my overflowing unsolicited phone calls.
According to Verizon's 'call forwarding' setup:
Both CDMA/TDMA configs are setup to use *71<number> as the setup code as this is provided by Verizon's own documentation.
So it doesn't matter which type you select to activate with. If you are having trouble activating HulloMail, please drop us an email at 'firstname.lastname@example.org' for privacy reason as we'll need your phone number to investigate your account.
This is sadly out of our control, as HulloMail requires the carrier to support the 'Call Forwarding' feature to work, if they do not support this then there is no way for users on these carriers to divert calls to our HulloMail Voicemail service.