1. Download the B025 firmware https://broadband.vodafone.ie/download/files/HG556a_B025.zip
2. Login to the modem Username: admin / Password: VF-NZhg556
3. Flash the firmware according to this website to vodafone ireland webiste http://forum.vodafone.ie/index.php?/topic/2111-huawei-hg556a-software-update/page__p__12008__hl__hg556a%20firmware__fromsearch__1&#entry12008
4. Open an voip account with any provider
5. Under the 'Voice' menu, you should be able to enter the voip setting.
Risk warning: This firmware is provided by vodafone ireland. Vodafone NZ may not provide any technical support. It may do permanent damage to the modem if not flash correctly. Make sure you understand the risk before you proceed.
Hope that helps! Enjoy!