Unfortunately, if you're somewhere such as the USA, the charges for PAYG are very high, and World Traveller isn't available to you. Also, be aware that receiving calls is only slightly less expensive, so that will also get through your balance very fast.
Your best option, if you have wifi available in your hotel, is to use an app such as Skype. If you have a BT account at home, you might find their SmartTalk app useful, though I'm not sure if you could set it up from outside the UK. (Edited to add that I've just set up SmartTalk on a second phone, and you definitely have to be in the UK, indeed in the house where the landline is, in order to set it up.)
Another option might be to get your contacts to call you at your hotel. Outgoing calls from hotel phones are often hideously expensive, but calling from a UK landline or mobile may be very much cheaper, depending on the deal your callers have with their network. For example, I'm opted into Vodafone International (I'm on PAYG), and calls to standard US numbers (from the UK) only cost me 1.5p per minute!
Depending on how long you're away, and what usage you require, another possibility might be a local SIM card, but for that your phone would have to be unlocked from Vodafone - again, if not already done, not feasible from abroad. Or even buying a very cheap basic phone could be an option.
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