Nextcloud
Nextcloud is a FOSS self hosted dropbox. Why use nextcloud? Because you want to backup files. Things like emails, contacts, texts, all can and will be lost on a mobile phone if you don't have a backup. Nextcloud can sync these up.
Setting up Nextcloud
You can do a self hosted server and manage it yourself, but then you have to manage updates. Another option is to purchase hosted and managed nextcloud services. There is a list on the official website. This is the way to go, I think. Especially, if you are an IT guy and deploying this for multiple customers. If it's just you, then go ahead, use an RPI and host it from your garage / basement.
Things to look for
Make sure the nextcloud provider you purchase from manages updates for you. Otherwise, you may as well just host it yourself, if they don't.
Keep track of where the servers are located. Most providers are EU at the moment, so that introduces latency for US users.
Useful Apps
There are phone sync apps for Android and Iphone.
There is video chat over IP:
You can have phone calls, with encryption. A step above a mobile phone call.
