you can get a sim from pretty much all electrical shops, dont forget your mobile must be unlocked to use it.
cheapest way to call the uk is going into phone kiosks which again are located all over and easy to find

Simple answer is DON'T. Telephone from street kiosks, do not use room phones in complex. 4 mis from street kiosk will cost 1 euro. if posing is required, just pretend you are talking to someone, then when the coast is clear, use a kiosk.
Enjoy your holiday

thnx for replies, think i'm past the age for posing , but will forward tip do my daughter and son.

can someone tell me if T mobile works in Tenerife

Be very careful if you have Orange as your net work, I was stung 2 years ago, text my son form Tenerife airport to say we had arrived, he had flown out a few hours before us, it cost me £4.00 the next day I phoned him I was on for a few minutes and then My balance was empty £ 20 in a text and call. Phone from the kiosks we always use them they are so cheap.

T Mobile will connect to another network, only problem is you will pay the spanish network you are connected to, as well as T Mobile in the UK..........but it will work yes

If you want someone to stay in contact with you, agree with them that they can ring the mobile and you can call them back from a payphone.

Why not just buy a Spanish Sim on arrival from any mobile outlet, it works out a lot cheaper