It always worked for me.
I have one for each devices: OBI110, OBI200, OBI202 and OBI1022 and each of them have one freephoneline account(paid SIP configuration) configured. I don't dial *98 to retrieve my voice mail, but the number for the line itself instead. After I listen to it, just press 7 to delete it. Never went to their website to delete it. After a few minutes, the light will go off. I also have an legacy IP phone (Linksys SP941) with freephoneline connected with no issues for the message waiting indicator. Dial tones will be back to normal at the same time when the message waiting indicator went off.