I wrote this as a reply to another message - but spent a bit of time on it - so felt it merited a thread of its own.
T mobile is most expensive and charges 40p for these.
We charge 20p
Asda is the cheapest at 8p, but currently buying a sim and £5 credit gets you £10 credit - so is really 4p
0845 numbers just point and a normal geographic number, which often you can find by using...
Which describes itself as "Alternatives listed for 0500, 0800, 0808, 0844, 0845, 0870 and 0871 numbers. "
So would offer you a number you can call for 8p or from your inclusive minutes - or in some cases free - as it will suggest an 0800 number.
Many government departments now have 0845 numbers, and you can burn through your entire top up on listening to how "your call is important to us" interlaced invariably with Vivaldi's four seasons. These numbers are are normally not listed on the site I just mentioned - but a Freedom of Information request will obtain the correct number. This may sound excessive, but it wont once you realise you have spent over a months council tax in credit trying to sort out errors in your council tax - and they are being paid revenue to keep you holding for their being incompetent.
The cheapest way I can see to call an 0845 number from a mobile is either using VOIP or a "local connection number" from a Betamax company. These make up the majority of VOIPcos and tend to vary their prices often - with the newest one appearing being best - but then worsening over time. So you need to keep moving to the newest or check the numbers you call, rather than sticking with the one you credited last time.
Using these services with GG also enables you to get better prices that Lyca Mobile for international calls from mobiles!
Here is an example of a Betamax co, and showing various ways of making a call (landline, VOIP, wifi, mobile, etc), but its worth noting that although Which? magazine found the service to work quite well, they had concerns over the lack of information on the services, and virtually no CS.
These VOIP services can be accessed directly on nokia smartphones, or using Nimbuz, Tring, or Lowcaller...