Avoid 087.. Numbers
A lot of people aren’t aware that numbers starting 087.. typically cost between 7p and 10p per minute during the day.
To avoid using these numbers, I suggest you go to www.saynoto0870.co.uk where you can read about how to avoid these expensive numbers.
Use your mobile or use your home landline telephone?
No ‘correct’ answer to this, it depends on how much and what you use your phone for. Personally I have moved to making nearly all my standard UK calls, 0800 and mobile call from my mobile. My ‘customer deal’ with PlusNet gives me 1000 minutes plus more text than I can use and 1.5GB data. The main reason for this move, is that most of my contacts now normally expect me to call their mobile phone, and this is the lowest cost way of achieving this. I can still use my PlusNet landline for many free evening and weekend calls. For international, I use 18185, see below, as I do also for 0845 and 087.. calls during the daytime.
Free weekend calls
I recommend switching to PlusNet (along with your broadband). Outside of this I have switched also to using Call18185.com (see below) for most other calls.
5p Local and National calls (i.e. 5p per call NOT per minute!)
Whatever your telephone supplier if you register with http://www.18185.co.uk then by dialling the prefix 18185 in advance of the telephone number you choose, you can get local and national UK calls of any length for 5p, any time of the day or night. Also using 18185 gives you attractive international call rates such as 1p/minute to the US or France any time of the day or night. Another attractive (to me) feature of 18185 is that they provide a voice announcement in advance of the call telling you what the per minute rate is - this can be a great money saver, particularly when someone gives you a mobile number and you ring it during the day, to find it is costing you 6p/minute (that is using 18185 - it can be quite a bit more with the likes of BT).
Updated October 2017.