I will be going to London and Paris for 10 days in the end on January (5 days in each).
Is there a sim that I can get with data and calling to the US that will work in both places?
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You have a few options available to you. The exact answer for you will depend on your expected calling and data use, so you'll need to price things up yourself.
Options #1 - two sims. Get a pay-as-you-go (aka pre-paid) sim in both countries. There are lots of operators in both countries who offer decent rates, and you can probably arrange to order online and have the SIM waiting for you at your hotels when you arrive in each country. This will probably work best for heavy use
Option #2 - UK sim + roaming. Most, but not all, UK PAYG sims will support roaming within Europe. Roaming rates are capped by the EU, so expect to pay legal maximum + 20% VAT. This will work best if you reach the UK first, and if most of your calls/data will be in the UK.
Option #3 - French sim + roaming. As option #2, but the other way round.
Option #4 - "Roaming" sim, from a European company. This would mean only one sim, and only one number, but likely higher call and data costs than a local sim. Will work best if you don't plan very much use, and want things to be simple.
You'll also need to consider if people will need to ring you - if so, you may be best off with option #1 so there's always a local number to get you on.
We've got lots of other questions on sim cards here, many of them will give you pointers / tips on each of these options.