flutter application for online Fax services

we need to develop mobile app (iphone / windows / andriod ) similiar to this https://apps.apple.com/us/app/efax-app-send-fax-from-iphone/id399922064 , we will provide you rest api for backend functionality means send fax / recieve fax here are sample rest api https://apps.apple.com/us/app/efax-app-send-fax-from-iphone/id399922064
please update with your estimate and timeline for said app