Advice please. I'm a VOIP rookie.

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Someone has asked me to contact them and has given me a choice of:
(a) Calling an 07017 number.
(b) Calling a Skype phone.

07017 calls are 35p a minute from my Virgin Media landline.

So I'm looking for a cheap way to call Skype.

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I've never used any VOIP phone service before and, preferably, I would
like not to pay for credits which I may never use.

Can one of the specialists here advise me of a cheap way to call a
Skype number.

Thankyou.