If your laptop has wi-fi built-in many sites have either free or low-cost access. Most McDonalds also provide free wi-fi and it is 24/7 and works outside. If you laptop does not have built-in wi-fi buy yourself a USB dongle; if your PC is older and only has USB 1.1 then get a PCMCIA USB 2.0 adapter to put in the slot, then plug your dongle into that.
If the laptop route is an option then either use Skype - which is free to another Skype user, anywhere - or open an account with a VoIP provider such as voip.co.uk or sipgate. You can load it with, say, a tenner, and it will cost you 2p/min maximum to a landline. Interestingly it will give you a UK number which people can use to call you. Download a soft-phone - I use Phonerlite - and buy yourself a USB handset, then it is just like making a normal phone call. I have found it very effective and the speech quality is as good as a line phone albeit with slight delay sometimes.
As others have said you can buy a PAYG SIM in France and stick it in your (umlocked) phone (French supplied phones incidently are all unlocked - it is against the law to lock them) and it will be cheaper then using a UK phone. If your phone is not unlocked you can buy a phone from Asda on the Asda tarrif (Vodafone - but you don't have to activate it) for £15 and us that over there. Whilst people calling you will have to pay more you will not have to pay to receive the call. The downside is that making calls does not keep you account active, only topping it up does that. Minimum top-up is €10-15 dependent upon provider, and if you don't top up the phone in a fixed time - with SFR that used to be 8 weeks - the phone stops working and you loose any credit. With some providers you cannot ring the top-up line from outside France other than on your mobile, so if it should expire then you are stuffed. The up side is that IME most of the top-up line do have operators that can speak some English.
Finally, consider text. You can make you phone go immediately to voicemail and change your outgoing to say only leave a message if it is frighteningly urgent or better still send a text. You will be notified if a message is left and can make your own decision how to respond. Receipt of voicemail notification and/or texts is free. Sending a text is more expensive than over here - upwards of 22p (€0.25) but at least you are not chatting and running up costs. You can also buy a French payphone card which costs (or used to cost) about 10p/min to call the UK - which is cheaper than mobile. Until it runs out of money a phonecard never expires.
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