New sim and new mobile
Hi Pete11!
Thanks for getting in touch today!
Our SIM cards will work in any unlocked handset! If your handset is part of a contract, then it's possible that you'll be unable to unlock your handset as you're paying for both the tariff as well. However, you'd be best checking this with Vodafone first!
If your handset isn't part of a contract, then you'll have to contact Vodafone and request your phone to be unlocked. Once you've unlocked your handset, you'll then be able to pop one of our SIM cards in!
In regards to the voiding of your warranty, most handsets come with an initial one year warranty, from the manufacturer. We cannot confirm whether our SIM cards will void this warranty, however, give your handset manufacturer a call, and they'll be able to best advise you.
The great news is, as an existing Plusnet Broadband customer, you'll be eligible to take up one of our exclusive Mates rates deals. These deals can offer up to double the amount of data of a standard tariff!
If you have any further questions, then please don't hesitate to drop me a PM, and i'll be more than happy to follow this up there! -Sean 
Hi there and thanks for the reply.
The phone I have is on pay as you go so no contract. I wasn't worried about the sim causing a problem, it's in case the phone develops a fault or has to be returned for some other reason. I know this would be covered under warranty, but I was worried about the warranty being void if the phone was unlocked. I will do as you suggested though and contact Vodaphone in the morning. 
I've found a thread on here that might answer your question 
https://community.plus.net/t5/Mobile/Unlocking-mobile-phone/td-p/1416342