Two things here..
1: "Not bothered about Volt" is fair enough, you need to choose the best supplier for each of your services, and I wouldn't choose O2 for my mobile provision either. But VM do use multi-buy discounting, and if it's cheaper to have a bundle with mobile than without - then no-one's forcing you to use the mobile SIM.
2: "Just want what we've got for a lower price" - you're effectively saying to VM "I want more discounts", you must therefore make it worth their while to do so. Find alternative provision for your services, at a price you are willing to cancel VM to achieve. Then offer to give your disconnection notice if they won't meet the price.
If you can't find that leverage, then it may be decision time for you.
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