Ick. I wish we could kill SMS/Whatsapp/Wechat/LINE already and just do everything over e-mail or some other cross-platform, multi-login protocol. I hate being tied up to one device to message people. I change devices on the order of minutes, move over the course of a day between a few locations that all have devices I own, and don't regularly carry or look at my cell phone if I am already staring at something bigger…
Better it works pretty seamlessly for people not using it because it just appears to come from a regular phone number to them. They just have to have that number also entered in their phone. That's one of the major hurdles to having a good replacement for SMS is that if it doesn't integrate into normal SMS there's a friction if Alice doesn't have a phone that can use the fancy new SMS replacement that Bob and Carol are hooked on.
edit: I see someone beat me to this suggestion.