According to The Register Roberto Cazzaro has said he has no idea whether the Microsoft Phone software is good enough to be accepted by the carriers.

This is to say that it may be unstable to the degree it will bring down a cellular network and therefore will not be allowed on the network. This decision is upt to cell service operators.

Register Article

You can also read the original IDG article which has the quote:

"This is the version 1.0 software. To be honest, I don't even know if the software is good enough for some of the carriers," Cazzaro says. "We already have been doing extensive tests but at the last minute the carriers can say 'No, no, you need to change this or otherwise we don't want to ship it.'"

and

Although Windows Powered Smartphone 2002 is nearly ready to ship, phones based on the software are unlikely to appear for up to six more months, as hardware makers ready their phones for mass production and mobile operators continue network trials using phones based on the software, Cazzaro says.