Vonage had to absorb yet another shot, this one from Pali Research, which issued a report suggesting investors should sell Vonage stock. In addition to current price pressure, Pali analyst Richard Greenfield said, things are going to get worse when cellular operators enter the VoIP landscape.

Pali noted several initial instances of that incipient trend:

T-Mobile has launched a service in Seattle that costs an additional $20/month for unlimited inhome (and at T-Mobile hotspots) Wi-Fi calling using a free VoIP router. The researchers note that’s $5 per month cheaper than Vonage’s product, and Vonage does not have a national cellular roaming option. 

In the U.K., BT Fusion has relaunched and now offers an integrated cellular/Wi-Fi service. Consumers that subscribe to BT Broadband and BT wireless can make calls over Wi-Fi at home and at BT hotspots throughout the U.K. There is no extra cost for the in-home VoIP calling, but, a consumer’s allocation of minutes will be depleted.

Apple and Cingular, meanwhile, announced the launch of the iPhone for June 2007. While the voice functionality is completely cellular at the moment, Pali Research notes, the device actively switches to Wi-Fi for Web-browsing when it becomes available. Given that AT&T (Cingular’s parent) offers a separate VoIP product (Callvantage), it is not hard to imagine the product integrating Wi-Fi VoIP (particularly as it has a fully-functional Safari Web browser that likely enables consumers to use applications such as Skype for VoIP), the researchers said.

All of that just adds to the ongoing risk to Vonage, Greenfield said, from competitors that can leverage two-, three-, and four-product bundles; such as AT&T’s recent announcement of a new plan that lets AT&T wireless subscribers make free calls to all other AT&T VoIP, landline and wireless customers.