Dylan Voller, the youth at the centre of the Turnbull Government's royal commission into indigenous youth custody, has had his phone privileges cut days before he is due to give evidence.

His mother Joanne Voller said the family had been told by Northern Territory prison authorities that Dylan would be unable to make phone calls for 28 days. Voller is expected to give evidence to the royal commission in a closed court on Monday.

Ms Voller appeared before a rally for Human Rights Day at Customs House in Sydney on Saturday with her head bowed and duct tape over her mouth. The crowd of several hundred, including many Aboriginal family groups, was told she wanted to draw attention to the way her son had been silenced.

Ms Voller had visited Dylan twice last week and left $150 for phone credits with his lawyer so that he could contact his family.