You can chat (not phone) to MSN users with the Nokia built-in google talk client, using a jabber account. From: http://www.jabber.org/user/userfaq.shtml:

"3. Can I use Jabber to chat with people who use other IM services?
You certainly can! Communication with legacy IM services is made possible by what we call "gateways". Jabber developers have created software for gateways to AOL Instant Messenger (AIM), ICQ, MSN Instant Messenger, and Yahoo! Messenger. However, not every Jabber server runs the gateway. See the Jabber User Guide for help setting up the gateways in your Jabber client."