Google made its second acquisition of the week yesterday, announcing that it has bought VoIP firm Gizmo5 for an undisclosed sum. The deal is designed to strengthen the search giant’s existing Google Voice application for making VoIP calls via PCs and mobiles. In its official blog, Google said that “Gizmo5’s engineers will be joining the Google Voice team to continue improving the Google Voice and Gizmo5 experience.” Google said that current Gizmo5 users will still be able to use the service, though new sign-ups have been suspended and existing users will no longer be able to sign up for a call-in number. “We don’t have any specific features to announce right now,” noted Google in the blog.

Google Voice – which was borne out of Google’s earlier acquisition of GrandCentral in 2007 – is Google’s answer to VoIP services from the likes of Skype. However, the application has been at the centre of controversy recently over allegations that Apple blocked Google Voice on the iPhone in collusion with AT&T (the iPhone’s exclusive carrier in the US). Google has also been accused of blocking some calls on the service to lower costs, giving it an unfair advantage over traditional regulated operators. Both issues are currently under investigation by the FCC, the US regulator. Google’s acquisition of Gizmo5 follows its US$750 million purchase of mobile advertising platform AdMob earlier this week.