Sorry, can't tell you off-hand the cost of calls to UK mobiles with Telefonica, (maybe their website has rates?) but once you have a line, you can use other providers for routing your calls, usually costing far less than Telefonica. Have a look at this one, for example: http://www.direct-telecom.es/normal-telephony-overview.asp
I haven't studied the website, but I'm sure you can find their rates somewhere. The other obvious alternative would be VOIP services such as SKYPE, although the variable quality may make it unsuitable for business purposes?
Hope this is helpful. As for your second question, probably would depend very much on what area you are interested in, but I'm sure you could do a lot worse than starting with the property rental pages on this site!