I've tried creating a Taxis G7 account using my U.S. Phone number (including the country code), but each time I get a message saying it's the wrong number. What do I need to do to enter the number properly or do I need to use a Europe-based number?

You need a French number. They use the number to, among other things, alert you when the taxi has arrived. They are not going to make a call to a U.S. number at their expense to do that.

Can I use a UK #? If not, they could text me as Uber does.

I don't know if a UK # would work. Never tried it. I have a French number. And normally they do text instead of call. Try the UK # and see what happens.

Starting in June 2017 there will no longer be any roaming charges anywhere in the EU. Then again, we don't know if the UK will be in the EU in June 2017.

Actually there will be no roaming charge in the EEA, so including Switzerland and Norway. Chances are the UK will keep the no roaming charges deal even if they vote to leave, which I doubt it will have happened by next June.

Texting a US number would also cost them more money than texting a European number, which as of today has become cheaper, just as calling and internet roaming have.

Yes, the cost went down something like 75% today for most Europeans.

(Who says the EU is not good for anything?)

Why trying so hard to create an account when you can directly call their number to book your transportation? You can also try http://www.eden-transports.com/airpo...les-paris-orly one of the best VTC companies I know in Paris.

How about a taxi from the queue?

The only choice is to use the phone number from your French hotel.
Failing that, go to the nearest cafe/hotel and ask someone to call you a G7 Taxi.
Or, if it's not urgent, look for a taxi on the street.

G7 will only text you to advise the make and color of the car which will pick you up - so it's not important if you can't answer. You should be outside waiting on the sidewalk before the appointed time, anyway.

FWIW, a belgian cell nr is ok with G7. It used not to be the case 2 years ago, but since they stopped discriminating me.