If you qualify for Volt benefits by linking an existing O2 account to your VM account then you'll get 1-3 pods for free, and they're well worth trying in that situation. if you don't qualify, then they're £5 a month, and you can probably order them via the telephone sales and upgrades route. Don't agree any other contract changes at that point - on their own the pods don't create a new contract, and the sales and upgrades team can't offer any decent discounts. At a fiver a month it adds up to the cost of a decent entry level mesh wifi system over 12-18 months. For me that doesn't make sense, but for some people having all wifi arrangement managed by VM seems a good idea, and it's usually the easiest option.
If you're going to be renegotiating your contract, then it should be possible to get the agent to throw in a couple of pods and a discount (if you negotiate well), although they'll be pushing you towards a Volt contract that includes an O2 sim. Might be worth trying the Whatsapp channel to negotiate any new contract, because it's safest to have a record of what deal you were offered and agreed to - make sure the chat includes that the agent confirms everything that's important to you about the new contract.
