iMessage to Android phones over WiFi
iMessage is Apple’s proprietary messaging service. There is no other cross-platform app that supports it. iMessage does not support calling as it is only a text messaging service. It provides its own peer-to-peer messaging that is available only to users of macOS, iOS (and by extension to watchOS). It also supports SMS for communication with non-Apple devices. Cellular calls can made using the iPhone’s phone app and calls to Macs and other iOS devices can be made with FaceTime app. iMessage is integrated with both, but does not do the actual calling.

If you have no cellular service, it is not possible to contact an Android device with iMessage, as it can only contact Android devices using SMS. (iMessage can text and call iOS devices with just Wi-Fi however).

You could choose other services that are available on both iOS and Android such as WhatsApp, Google Hangouts, or Google Allo. Generally, both devices must have the same app installed for communication between them.