WhatsApp was bought for $19 billion. You could probably pay $1 billion and get a couple of different systems that worked well enough to replace WhatsApp.
Having read the engineering posts about their core systems I think you're being wildly optimistic in the same way as people who think "writing a general CMS can't be that hard, I'll just write my own" are.
You could build 17.2 WhatsApps for $1 billion, assuming that it took nothing more and nothing less than the $58 million in funding they raised: https://www.crunchbase.com/organization/whatsapp#/entity.
Yup, that's why I said a couple. You could afford 17, but only 2 of those would be suitable.
most likely their internal system can only think in telephone numbers or something like that