I have lived back and forth between Europe and the USA over the last few years. Communication is always the most difficult part of the move. It's a reminder of how far off our tech is from the true ideal. In Europe, everyone has WhatsApp. For a while, this was great! One app, one "to reply" list. A taste of what messaging could be. I quickly found myself frustrated. WhatsApp relies on phone numbers, which muddies up…
1) Port your US number to Google Voice for a one time fee of $25 ; from now on, you can use this number to forward to your new verizon number (also: T-Mobile had much better plans for only a few months at a time, every single time I checked).
2) Get service in Europe from an Illiad affiliated phone company; At least in the past, that included free US numbers you could forward your US google voice to freely -- as well as 15-45 days of free US roaming service. Maybe these offers are gone now - you could just pay $1/month or so to LocalPhone or a similar company to do that forwarding for you.
3) Buy the cheapest Android handset running a recent version (for security updates); I got a new Meizu m6 for ~$90, you could probably go lower for 1st hand or get a 2nd hand at $40 or so.
4) When travelling, switch SIMs between main phone and cheap phone, set up forwarding (directly if included in plan, or through LocalPhone or similar if not), and keep cheap phone on WiFi only, plugged in charging at home, and mostly accessible for when needed.
5) Use WhatsApp web on the new phone to continue using old WhatsApp number. Less than ideal - messages don't pop up - but it works well.
For about $100 one time[0], and about $1/month or so, you could to keep both numbers fully functional and working indefinitely.
[0] Ok, so you'll have to upgrade the cheap phone for security reasons every few years. So not exactly one time, but possible $20/year amortized, or free if you don't trade-in your old main phone and just let it rot like many people do.