My experience is that other providers (Jazztel, Ya.com, Orange) have had split success in penetrating the market. According to the testimonies I've heard, service has been so and so and the infamous 20MB lines never deliver what they promise. Some of these companies may now have a proper network as opposed to renting the lines from Telefonica, but I would guess that in the Marbella / Nueva Andalucia area these providers still rely on Telefonica.
I've been with Telefonica the last 8 years and that has worked out just fine. No major outages, except during the first year of service, and support has been OK. The service is pretty expensive for what you get, but improvements are on their way. I live in the upper reaches of Nueva Andalucia, so I'm still stuck with the not so decent 1MB service, but if you are closer to the N340 road, 10MB should now be available and for just a few Euros more - 44 I think, whereas I pay 40. You have to pay a monthly rent of the line, around 15 Euros I think, and IVA (VAT) on top. In my case it amounts to roughly 65 Euros a month.
Telefonica have been known to show little interest in expanding their lines inland, so if their service is unavailable in your area - and believe me, you don't need to be very far off the N340 to not qualify - check if there are any companies in your area that do wi-fi internet solutions. Yes, with pretty powerful antennas with a range of up to 30 km, you can easily get connected with decent speed, and it will not cost you much more that Telefonica. Some of these providers also have short-term solutions, so if you're here only for a few weeks at a time, you should basically only pay for that period. Depending on how far you are from their antennas, you can get connected either by attaching a signal booster to your computer (usb port) or if you are further away, a better solution would probably to install a small antenna.
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