Yesterday our provider [Speedlinq, or Telfort Internet or GreenISP or whatever they want to call themselves nowadays] switched to another VoIP platform for ADSL connections. They sent a letter with a cd-rom for changing the settings in VoIP-enabled ADSL modems, but we have a VoIP phone, with SIP routed through to it. No problem, their help site had settings. Except that they didn't work and did not map well on the configuration fields of our SIP phone - an E-Tech.
So after fiddling around, I discovered I should just ignore the sip.telefoniedienst.nl server completely, and only use tel.telefoniedienst.nl:
Domain: tel.telefoniedienst.nl SIP Proxy: tel.telefoniedienst.nl:5060 Phone number: 10-numbered phone number, like in the letter, starting with 0 Account: Same number Password: Provided in the letter
... with domain being the SIP server [registrar/realm] you log into. This worked! See the complete settings for the E-Tech VPPH01 VoIP phone [anonymised a bit, be sure to set the correct IP addresses etc too].