I **think** Vonage have a UK based proxy, which will be substantially closer to Israel c.t a US based one, talk to Vonage support and see if they can set your a/c to point to the UK proxy.
I don't really see that being the problem though, I used to be in Cairo, Egypt until just over a year ago. Initially used Broadvoice until their meltdown and despite BV being US based, never had an issue with sound quality. I then switched to a UK service provider,
www.voiptalk.org and used them w/o issue for quite a bit as well.
To eliminate ISP issues, sign up with a pay as you go provider (like VoipTalk.org), configure them on an ATA (if you have an unlocked device, or get one, SPA1001s are quite cheap and work wonderfully), or a softphone like X-Ten, and make a few calls and see if they're any better.
If even with a different provider gives u the same sound quality problems, then the issue could be your ISP, or congestion in your internal network (someone using torrent??)....