SIM Card Situation

1. I am a Vodafone customer, however any SIM should work in both places. There should be no roaming charges within the EU.
2. Vodafone works, but any provide should allow you to do so. The issue is that it costs more, therefore you need to have a plan that includes international calls/SMS
3. This is probably not possible. You will have a U.K. Number, therefore any calls to that number from the US will likely incur international calling charges based on the pricing of the long distance provider used by the US caller. Nothing is free in this world.
4. Check the websites of the major UK mobile phone providers, including Vodafone. They should have all the plans they offer. You can google major UK mobile phone providers to see which they are.

Who is your cell provider here in the US? Many of them have one month plans for calling/texting abroad that might work if you don't need to always reach out and touch someone or constantly update your status on social media.

I don't want to be so off the grid that I have to be tethered to a wifi connection in order to be in contact

If you're that far off the grid not to receive a phone signal you certainly won't be getting a wi-fi signal.

You're going to be charged for international calls irrespective of what plan you opt for.

I think Roberto's reply is pretty accurate, so I won't repeat any of that. Most SIM cards ARE pay as you go, but some do offer in terms of minutes, texts and data amounts. Have you looked at Carphone Warehouse's website? They have the ability to compare brands. Though, if you are going to be in both UK and Ireland, I do think Vodaphone or 3 might be your best bet due to coverage.

Secondly, I'd focus on the data part of the bundle because the apps that you speak about are actually your best bet for your contacting your family in the US without extra charges. You don't need to be connected to Wifi for them to work as most of them actually use data. In the 3G-4G world, there are few instances where you won't have data but be able to receive calls/texts. If you have an iPhone, and your family/friends also have Apple devices, then iMessages (which works on data) will still work and allow you to "text" using data, not texting allowances. Likewise, you can use audio or video FaceTime to make sure your calls are using data. Other alternatives include the Facebook messenger (which is both messages and calls), GroupMe, WhatsApp, GoogleTalk, etc. I have used these options on multiple trips without any issue. I simply make sure I buy at least a 1G data plan.

I vote for adding WhatsApp to your phone and encouraging family and friends to do the same. I use here in CA to text and send photos to friends in the UK and Eire. And when I'm in Belfast, London and Dingle vice versa.

Roberto has provided a succinct explanation and YES, Ireland is another country and separate from the UK which encompasses Norther Ireland. In fact when in London watch the weather report. You won't get any news of what weather will be like in Eire.

I have Verizon, and while I have looked into the plan options, none of them are quite what I'd prefer. The most all-inclusive plans is $10/day which for 2 weeks is really, really pricy. The other plans are...okay, but not great. But, at least I'd be able to keep my number. You can get 100 minutes of phone time plus text, or you can pay as you go, but either way, you have to pay for 100 MB increments, which is virtually no data, and would add up really quick. I think I'd be better off doing the SIM card thing.

It looks like Vodafone has a 30 day plan, where you can add 100 international minutes for 3 pounds more. That may be the route I go.

My parents aren't hugely tech forward. No smart phones for them, no iPhones, so iMessage won't be an option. Sister uses a Droid of some sort, but not Apple. My Mom does have an iPad, which she could load some apps onto, or we can converse through Facebook, but she isn't going to have it at all times, and if something were to come up, I doubt she would have it with her.

I figure I will give them the UK phone number I am assigned, and if they have to call me, they have to call me. (Hopefully, there won't be an emergency or anything). Otherwise, I will rely on Facebook or another message app of some sort.

If people in the US want to contact you on your UK phone, they can text you; that's much cheaper than an international call (usually about $0.20-0.50 per international text). You can then call them back; many UK cell phone plans have cheap calls to the US, and certainly cheaper than in the opposite direction (it can be $1-2 per minute to call a UK cell phone from a US phone).

If you sign up for google gmail, you can get a google voice number. It's a US based number xxx-xxx-xxxx. Get the google hangouts app and set it up. Calls to the GV number should ring through to your phone as long as you have cellular data or wifi. This should make it easier for your parents and others to call you.

I haven't tried it myself but seen many accounts of it working. You should probably test it out at home prior to leaving for your trip.

I have an unlocked iPhone 7+. At Car Phone Warehouse I bought a SIM card July 9 -17 in Bath but it's a national chain store related to Best Buy US store. I paid 15 pound for MTV TRAX SIM -7 GB -500 minutes -unlimited text I paid cash no personal information unless you want to top up. The store said the card would not work in France but it has worked great better than wife's Verizon plan better coverage from many providers. I can not call or text the US unless using iMessage or emailing both have worked with pics included at full size. The card last 30 days until needing to be topped up. I would try this card before buying a second SIM card in another country. This card did also work in Wales This was the best card available at the store the other cards had less data I also had service in York. That is as far north as we made it. You can receive calls on it from anywhere but I'm sure the person making the call will pay extra to your U.K. Number. The card was active in about 30 minutes. You can not call the US Family but they might be able to upgrade there cell plan for a short period to include international calling. We did with Verizon before are trip to call for reservations & other information & cancelled it before we left. Verizon does have a plan for $10 per day that`will allow you to call any where as well as receive calls & use data with out changing your number while in Europe. Let them know in advance. The wife has been using that plan it is pricey but service has been good but mine with the SIM card has been as good

Linehand1, that is a fantastic reply, thank you.

I've just signed up for Google Voice for me and my wife who will be in the U.S. when I'm in the U.K.

I checked on the Car Phone Warehouse site and also found the EE MTV Traxs at GBP15 with 7 Gb of data plus U.K. calls plus U.K. texts is the best buy.

Thanks everyone.

Yup, Verizon gets you one way or the other. You end up paying an obscene amount per day or you can do a more ala carte thing, but it is very limited on what it offers. There is unfortunately no middle ground.

I think I have made peace with the fact that I am not going to get everything I want, and will likely be restricted to where and through what I can contact my family. I mean, with the time difference, they will have an idea of when I will be in contact, and I admit it seems kind of silly to focus on a worst case scenario/emergency situation. I just know...actually multiple people, who have gone on vacation, to Florida or wherever and had family emergencies where they have had to return home.

Normally, I travel WITH my family. This would be a rare occasion that I would be traveling apart from them, and I guess I am just...planning for the worst. Which is never a good way to start a vacation.