Just had a letter from Virgin Mobile with changes to charges. (I'm on the £8 a month 100mins / 100 texts tarrif but this may apply to others)
Calls to Mobile networks outside allowance down to 35p a min from 40p a min.
UK landlines and other virgin mobiles - flat rate 20p/min instead of 15p for first 5 mins and 5p/min thereafter.
You will be charged for one full minute for each call you make or one full minute from your allowance with per/second billing thereafter.
If you don't lke these changes (I certainly don't) you have the right to cancel your contract.