If high costs are likely to be incurred when working with your client then we would recommend that you discuss with the client a suitable way to cover such costs, for example, provide you with postage materials, a credited skype account for calls etc.
(Remember - Time etc will not cover the costs so make sure you remember to organise this with yourself or the client, which is your responsibility as a freelancer)
United Kingdom
If phone calls are going to be made just within the UK and you have an unlimited minutes contract then we would recommend to simply use that rather than trying to arrange a credited Skype account which could potentially delay the work and support you are able to provide your client.
Tools we recommend to allow you to make calls for clients
Google voice is a free to use tool for calls within the USA & Canada.
If you're based in the UK and your client is requiring you to make outbound calls on their behalf then you are welcome to suggest that they set up a Skype account which is credited by the client.