#1 - never call 0870 / 0845 numbers, either on my landline or mobile 'phone. I always check on SAY NO TO 0870 and find out what the company's landline number is. Just type in the number or the company name and you can usually get their landline number.
Depending on what call plan you are on you will usually get charged for calling these numbers and they can be expensive. My mobile provider charges me for calling them whatever time of day, despite hundreds of 'free anytime minutes'. BT will also charge you to call them during the day.
#2 - I also use my mobile to make any calls during the day to use up my call allowance and cut down BT costs. I have just received my latest bill from BT and I have spent £0.87 on calls (actually SM did as I have been disciplined about using my mobile minutes) compared to £13.76 on calls during the last quarter.
I am hoping for a reduction on my monthly direct debit in due course.
I have another 7 months approx' to run on my current mobile contract, for which I pay the princely sum of £25.00 a month (oh the shame!) I have a nice Blackberry and hundreds of 'free minutes' and mobile internet time that I don't use up every month. So come the end of the contract I will be keeping my Blackberry but reducing the call plan / contract to half of what I am currently paying as it is a WASTE OF MONEY.