New to using dual SIMs for travelling - Best practices?

In general you can configure in the Android settings which SIM shall be used for calls, SMS and data (similar to described in this article).

From what I understood of your post a Spanish eSIM with a pure mobile data tariff would be enough or something like Airalo.

Important: If you switch on your mobile phone in Spain with activated Verizon SIM and activated roaming you will receive a SMS from a Spanish provider about costs of services. This information message is free of charge. It can happen also when traveling in Spain because the Verizon SIM switches the roaming provider automatically.

Be aware that if you accept calls or open any SMS to your Verizon number, you will pay money.

Be able to continue to use my Verizon number for SMS and phone calls, but via WIFI CALLING ONLY.

This is wrong understanding of technology. Wifi is NOT a mobile network service, so mobile calls and SMS by "using Verizon number" is not possible. Wifi is a kind of radio standard on which IP or P2P based services can operate services such as calls or messaging. Sometimes also the name WLAN is used for what you know as wifi.

One more thing: if you are connected to an open public wifi network do not use any security related apps or services such as banking. For such things deactivate wifi (just to be sure) and use eSIM mobile data which is way better secured. Another option would be VPN but I do not want to overstress here.

Hope this helps. Good luck.

As a Samsung (Android) phone user, I've found I have to fiddle with my phone a bit so I can send outbound texts using my native (Google Fi) SIM. Google Fi doesn't charge for either inbound or outbound texts, so I want my texts to work as usual. If I simply install a data-only eSIM and activate it, I continue to receive inbound texts but cannot originate texts. The phone tries to use the eSIM for texting, which doesn't work because there's no attached telephone number. For each person I want to text, I need to switch to my original SIM within the texting function.

Others with free texting on their original SIMs should do a bit of Googling to learn what they need to do if they want to retain the ability to text via their original SIM. I think most Android phones have a single setting that can be changed; unfortunately, Samsung has "enhanced" the Android software and made it a bit trickier.

Use WhatsApp instead for calls and text using data - assuming you'll be communicating with people you know that will also use the app.

For better privacy use either Signal or Wire.

Key goal: AVOIDING having any Verizon charges due to connecting to a
mobile network for calls, texts or data!

as far as I know, on an Android phone if you want to use your Verizon number for calls and texts you have to use the Verizon SIM card and thus incur international charges - the phone number is embedded in the SIM. If you figure out a way around this, please post in the forum, I would be very interested to know how you did it. You can use the esim just for data and turn off the Verizon SIM for that purpose - it's in the settings.

I got a Google Fi eSIM and use it for data and as my travel phone number for airline/train/hotel reservations, banks, and share with my contacts that I will be communicating with on the road. I set up international pass with my carrier as a plan B but turn that SIM off when traveling. It is cost effective compared to my carrier's international plan.

Yes, here is how you can call out and receive calls you your Verizon SIM without incurring charges.

Get wifi calling working before you leave.

Then keep your phone on airplane mode. And make SURE you don't accidently turn it off. I moved the button that turns it one and off so I wouldn't do this absentmindely.

This worked for me, when I just had a single SIM card and didn't get a local one. I just used wifi at the hotel, etc.

It doesn't work for SMS though--not even sure why I asked.

Wifi calling CAN work with texting. It's up to your carrier. Not all carriers, and not all cellphone plans allow wifi calling, and some charge for using it while roaming. You really have to find the correct answer for your particular plan and carrier.

It is not independent of your carrier at all. It just means that Tmobile, for example, allows your phone to access the Tmobile network through a wifi connection. Your text goes through the internet via wifi, to some Tmobile server, and then gets sent to your friend's phone in Chicago (using Tmobile).

A low cost MVNO may very well decide not to spend the money to provide that sort of service. Or they may choose to only allow voice calls (as the OP says). Or they may provide the service, but charge a few cents for each text or call if roaming. A while ago there was a thread here about Verizon charging someone a ton of money for making a lot of wifi calls from Europe. Maybe they had a really low cost Verizon plan and didn't know about add-on charges...

When I had an AT&T account, I used wifi calling in Europe to text friends back home for free. Now that I have Tmobile, I also text friends from overseas with wifi calling. A few minutes ago, I enabled Airplane mode, turned wifi back on, and texted my wife. No problem. In all cases, the text came from my "normal" cell number, and replies came back as normal, and I used my "normal" texting app.

If your phone's hardware is very old, it may not support wifi calling.

NOTE: The above stuff refers to "wifi calling" as selected on your celllphone's options. There are other ways to text or call, usually for free, using wifi, for example Google Voice, TextNow, Skype, TextFree, and many others. These apps do not involve your cellphone carrier, and the call or text does not come from your cellphone's phone number.

Travel-Lite, there is a risk in your approach because you will not be able to receive any SMS with activated airplane mode, means also no local warning SMS in desaster cases.

Therefore I recommend travelers not to do it this way because European countries use the SMS alarms to all mobiles currently registered in affected cells.

this just popped up in my in-box

https://www.cntraveller.com/article/best-esim-international-travel?uID=7c410f027f87c20bc98af6219567d1e40ba4b2d84c89ac7381414d37bc095e5c&utm_campaign=newsletter_daily&utm_source=newsletter&utm_brand=cnt&utm_mailing=CNT_NEWS_Daily_150225&utm_medium=email&utm_term=CNT_DAILY_ACTIVE

Your Verizon phone will automatically unlock 30 days after purchase so you have the option of purchasing a European/Spain eSIM. Some only have data capability and others have data and call/text capability. I have been satisfied with Orange eSIM that has data, call/text capability, Other eSIM retailers have data only. Airalo and Holafly are a couple of companies that sell eSIM delivered to your email. Link below has other recommendations to purchase an eSIM.

https://www.traveltomtom.net/destinations/europe/spain/best-esim-cards-for-spain