This stuff generally does work - so long as it’s properly configured.
Are you saying that the phone is telling you it can’t see the SIP server?
In that case, you will have to check your phone’s configs. Have you configured the SIP server address as a domain name or an IP address? Have you got anything configured as domain names? If so, have you got the DNS settings in the phone configured properly?
Have you got a tftp server configured in the phone? My phone seems to hang before registration if there’s an address for a tftp server but there’s not server there etc. It’s probably a good idea with these phones to have a tftp server running on your local net somewhere (probably on the Asterisk server) but with nothing in it’s public directory (unless you’ve got things you want to put in there, of course). My phone certainly seems to get registered quicker if that’s set up.
This stuff probably isn’t the issue if the phone works ok with another phone though.
What happens when you boot the phone up?
I’ve had a couple of weird periods with my phone - similar to what you’re describing probably. It was kinda like the phone had got confused. It’s only ever happened when something’s changed - never just spontaneously when it’s happily registered. Last time i was pulling my hair out and i went through all the configs and redid them - including stuff on the phone LCD/keypad. Eventually it just started working again.
The annoying thing about configuring this phone is that field in the config web pages come up blank when they’re really configued with something. You can’t see what’s actually configured and it looks like nothing is. To make sure a field’s blank, you should type in some characters and then erase them and then save it. That could be just my web browser(s) (opera and firefox) but i doubt it.