Jurors in a US court decided Vonage infringed the patents of Sprint Nextel and awarded Sprint £34.75m in damages.
Last year, Vonage lost another patent case in a US court when it was found to have infringed the patents of Verizon and fined £31m.
It is appealing that judgement and is set to also appeal the Sprint judgement too.
To carry on trading in the US, Vonage developed network workarounds following the Verizon judgement to avoid using the patented technology in question.
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It will now have to do the same thing following the Sprint judgement. The legal cases do not affect Vonage's operations in the UK, although the financial collapse of Vonage in the US would.
The two damages awards are in danger of sending Vonage over the brink, particularly as its subscriber base is now slowing.