3 mos in Sevilla! Return trip, some tech questions

Jadzi,
The maximum time allowed in the Schengen Area for non-EU/Schengen citizens as a tourist without a visa is 90 days, not 3 months. A subtle difference.
You are only allowed in for 90 days as a tourist, You are not allowed to work, if you are working you need a visa from day 1. If they catch you working, you are an illegal immigrant, and they are liable to fine you, tax you on what you have earnt, and expel you with a ban on returning. It is your decision whether you want to risk breaking the law.

Yes, if your phones are unlocked and compatable get 2 x Spanish SIM cards for day-to-day use and as a backup to call the USA (WiFi out).
Make sure you have plug adaptors for all your kit, and that the power supplies and chargers can take 220V 50Hz.

We don't offer legal advice, but can say pretty confidently that telecommuting at a job that is based outside of Europe, which sounds like your plan, is not the same as "working in Europe." Similarly, US residents can travel to Europe to conduct business in person for their companies. Rick Steves and his staff do this all the time. But you can't be paid in Europe, or take work away from a local resident (even as a volunteer).

Here is a list of free wifi hotspots in Seville. Many are funded by the city or available in a business if you buy a cup of coffee. www.exploreseville.com/map/seville-wifi-hotspot-map.htm You can print the map and list and carry along with you.

Since you sound like you will either be there working for your own USA employer or as self-employed and telecommuting, there should be no problem working there. Penalties only apply if you are working for a Spanish employer "under the table" there or operating your own Spain based company without permission. I work in Europe and Asia for most of the year (for US based business) with no problems. I always have visas to the different countries so can stay most of the year. The 90 day limit only applies if you are a tourist with no visa. Visas change everything.

I agree with Laura, that unless you will be receiving payments issued by a Spanish or EU company, you really aren't working in Europe in any sense that you'll need to worry about. You're taking a laptop to the beach, essentially - I doubt anyone would have a problem with it.

I'm facing a similar situation (a month in Barcelona, working remotely). I've heard that European SIM cards usually have decent internet speeds, and are much more lenient about tethering - that's an alternative to Wifi. Note that there is a difference between GSM and CDMA though - be sure your current phone is GSM-compatible. I'm taking a "burner" phone instead - that way if (when) it gets stolen, I won't care much.

I've been doing a lot of research on the VoIP front - Google Voice & Skype are solid options, but there's also Vonage Extensions (if you have Vonage) and Obihai, and a totally free option called RingTo. I've been able to get it to work over Wifi on my mobile phone - not perfect, but it works in a pinch, and you get to own the phone number permanently, without cost. Requires a little legwork, but it's a nice option.

Have a great trip!

You can also use MagicJack to make free calls - Voip.

Seville is a very sophisticated city. Internet speed is pretty good there. You can also ask you landlord to call the internet company and request a faster speed with a wider bandwidth and you will pay the difference. (if there is one) She will need to be the one to call them though, just like in the US. They will want the person whose name is on the account to be the one who requests the change, besides, this transaction will all be done in Spanish, and don't know how good your spanish is!