I am also investigating options for an alternative to a cell phone number. VoIP.ms sounds like it might be a good option. The website says SMS is only available with certain numbers though and it is still beta. I like though that you can send your SMS messages to an email address.
How about Linphone, has anyone used that? You don’t get a phone number, only SIP address. The website says it also has encrypted messaging.
I have jmp.chat as well. It does work well for sms, but understand that the xmpp messages are not encrypted at this time. I don’t use it to make voice calls at all, I just intended it as a number to give out to businesses I don’t want calling me. It does have voicemail and sends you a transcription when you get one. It’s not too expensive, about $35 per year.
Also an option for iPhone users is the MySudo app. It gives you a real phone number with SMS. The lowest usage plan is $1 per month.