This is an odd little device. It seems you can only call other express chat users? it's reminiscent of cellphone systems from the old old days like Nextel walkie talkies. Also I see its available in India only and given the feature restrictions perhaps it's meant as an ultra low end device ?
If HMD came out with something that was optimized to the nth degree for whatsapp calls, that might be interesting, but even then I wonder how cheap it would need to be to compete with budget androids.
Several carriers offer this, usually aimed at corporate users (construction, maintenance, fleet workers, emergency responders). See Kyocera and Sonim, particularly their rugged phone offerings (Dura from Kyocera, XP from Sonim).
Carrier-based PTT is typically limited only to other phonesets on that carrier, though in practice the feature is generally accessed through a programmable key, and can be configured to use another (unlimited) press-to-talk app, such as Zello (which I've seen before) or Express Chat (#TIL).
If HMD came out with something that was optimized to the nth degree for whatsapp calls, that might be interesting, but even then I wonder how cheap it would need to be to compete with budget androids.