I've been using this PAYG system for years with a USB handset plugged into my laptops:
VoIPCheap | Free Calls from the UK (They also trade under at least four other brandnames with similar services)
Works quite well, though up until now it wasn't SIP compatible - you had to use their bespoke software
However I just noticed that they do now have Android client software, and they are now SIP compatible. So it should be possible to use it on a smartphone connected to a wireless internet connection. Far easier than messing around with Vonage. From what I can see the only part of Vonage which works with SIP is the Softphone Mixture pointed our earlier.
Considering how big Vonage are in this business, they really should do better