What SIP client/ ip phone are you using? While most configure up the same way, I've seen some phones that like things a different way.
Try with the domain name as the server name and as you've done, set the outbound/sip proxy to your server IP.
For example, using the Zoiper SIP client, we use the following as our basic config:
Domain: [your multi-tenant domain name]
Username: [extension]
Password: [password]
Use Outbound Proxy: [ticked]
Outbound Proxy: [your server IP address]