Cheapest phone packages

A lot of my work is done from a home office, I use Skype (using a London phone number) for calling landline numbers and generally my mobile for calling other numbers,

On the basis that most of my phone calls are to mobile numbers and HMRC on their 0845 numbers, I'm finding that my call costs are getting more and more expensive (not covered by either my Skype package or my mobile package).

What do other people use to call mobiles and 0845 type numbers?

By way of comparison Skype charge approximately 15p per minute to mobiles and between 7p to nearly 14p per minute for 08xx numbers.

I'm finding that calls to HMRC are typically lasting 20 minutes (once I've got through their numerous menu's and recorded messages, waited, then spoken to someone, then waited, then spoken to someone else, then waited for their computer system), before I know it the call cost is over £2.

Interested in knowing what others do?