It depends on what you want to accomplish. WhatsApp is a full featured messenger service that allows texting, voice calls, video calls and more between WhatsApp users. As far as I can determine, it is not compatible with standard cellular voice calling or SMS texting. It uses internet data so a data plan or wifi must be available. WhatsApp uses the same number as the cellphone plan provider to create a username.
If the aim is to communicate with any cellphone user, not just WhatsApp users, there are a number of apps that that support standard VoIP and SMS texting over a data plan or wifi. I use TextNow but there are lots of others available. These apps usually supply a separate phone number which is used in place of the provider number when using wifi. The main issue with these are possible poor voice quality and loss of phone number if not used regularly. Some charge US$10/yr for extra features or long term phone number assignment.
WhatsApp is considered to be a major disruptive force in the cellphone services market. WhatsApp is known to actively block users of other, similar apps such as Telegraph which attempt to be compatible. With over one billion WhatsApp users, it has been suggested that may be anti-competitive.
WhatsApp is owned by Facebook. It collects user data, such as messaging or phone contacts information, and shares it with Facebook for advertising and other uses. Although Facebook claims to have stopped such practices, that information may be accessible by other companies.