Google on Thursday said it had acquired Internet-based calling service Gizmo5, validating an unconfirmed report about the deal on Monday.

That was the same day that Google revealed its agreement to buy mobile advertising network AdMob for $750 million.

No price was disclosed for the Gizmo5 deal.

Gizmo5 makes software that allows users to place VOIP telephone calls using a computer or mobile phone.

In response to a query from the Federal Communications Commission, Google last month said that Google Voice should not be subject to the same rules as traditional telephone companies because, among other reasons, "Google Voice is not an 'interconnected VOIP' service" under FCC rules.

Google's ability to sustain its argument that it isn't a telephone service provider could be weakened if Google Voice gains capabilities like VOIP calling.

But that may be inevitable if Google's aim is to compete with Skype.

Google Voice product managers Wesley Chan and Craig Walker said in a blog post that Gizmo5 signups have temporarily been suspended.

"While we don't have any specific features to announce right now, Gizmo5's engineers will be joining the Google Voice team to continue improving the Google Voice and Gizmo5 experience," wrote Chan and Walker. "Current Gizmo5 users will still be able to use the service, though we will be suspending new signups for the time being, and existing users will no longer be able to sign up for a call-in number."

The pace at which Google acquires companies appears to be accelerating, as promised by CEO Eric Schmidt during the company's October earnings call.

The Gitzmo5 acquisition is Google's fourth this year. In August, Google bought video company On2 Technologies. In September, Google acquired reCAPTCHA, an anti-spam security company. And Google's purchase of AdMob was announced earlier this week.

InformationWeek has published an in-depth report on smartphone security. Download the report here (registration required).