Union leaders have accused the body, which looks after some of Scotland's greatest buildings, of eroding employment rights and paying "poverty wages".
The Prospect union is calling on NTS members and the Scottish Government to back its call for better conditions.
The Trust brought in zero-hours contracts, which guarantee neither hours nor pay, for the first time this year.
The NTS insists that it uses the terms sparingly and feedback from staff and managers indicates that they have "worked well".
Of the 1300 staff who worked with the Trust over the summer, 83 were on zero-hours contracts.
The union has accused NTS of trying to erode a system whereby many seasonal staff returned to the NTS year after year.
It wants NTS to pay all staff at least the Scottish living wage - £7.45 an hour.
An NTS spokesman said that in recent years the Trust had invested "significantly" to improve pay for its staff and was committed to phasing in the Scottish Living Wage over four years.
On zero-hours contracts, he said: "Feedback from both staff and managers indicates that this has worked well over the summer providing extra support for properties at busy times."
l Organisers of the Mobo music awards in Glasgow have been criticised over the use of unpaid volunteers to prepare for next month's ceremony.
General secretary of the union Bectu Gerry Morrissey said the organisers of the Music of Black Origin awards were "steeped" in equality and should not be "exploiting" people.
Mobo said that in Glasgow it had been working with Young Scot to provide opportunities for a small number of young volunteers to learn valuable life skills.