Signal
Pretty safe
Sending messages through Signal is pretty safe. There is also the possibility to erase messages on the sender and receiver's devices after some time. If you want to be more sure you can also buy a new SIM card that can be used on the Signal app.
What you should do:
A. Download Signal from the Apple App Store or Google Play for Android.
B. Install the app on your phone and create an account.
C. Select new message and enter the number of Lost in Europe:
+31 6 222 63 260
D. Type and send your message