The next time you've got a friend over, set an example and put your phone on the table, visibly there but not too far away, to let them know that you're intentionally present, not distracted, your attention is undivided and you want to be fully focused on being right there to spend time with them. Don't mention it or draw attention to this, you're not doing this to be preachy or wanting praise, you just want to be a good friend and you value your friend's time. Ideally, your friend will either notice this or even pick it up without conscious notice, and set their own phone aside on the table as well.
Then, when your friend takes a minute to go to the bathroom, grab your phone and take a photo of your friend's phone sitting on your table. Do not touch it, and put your own phone back exactly where it was immediately once you've got the picture. Carry on with whatever you two were doing.
Once your time is up and your friend has left for home, wait for a good 15 minutes or so, for them to either get back home or be well on their way there. Text your friend, "hey, you forgot your phone", and send them the photo you took of their phone on your table. Set a stopwatch running from the moment your friend sees the message.
Measure how many seconds it takes for your friend to process this and tell you to go fuck yourself.
