  (Sub) mm VLBI observations are strongly hindered by limited sensitivity, with
the fast tropospheric fluctuations being the dominant culprit. We predict great
benefits from applying next-generation frequency phase transfer calibration
techniques for the next generation Event Horizon Telescope, using simultaneous
multi-frequency observations. We present comparative simulation studies to
characterise its performance, the optimum configurations, and highlight the
benefits of including observations at 85\,GHz along with the 230 and 340\,GHz
bands. The results show a transformational impact on the ngEHT array
capabilities, with orders of magnitude improved sensitivity, observations
routinely possible over the whole year, and ability to carry out
micro-arcsecond astrometry measurements at the highest frequencies, amongst
others. This will enable the addressing of a host of innovative open scientific
questions in astrophysics. We present a solution for highly scatter-broadened
sources such as SgrA*, a prime ngEHT target. We conclude that adding the
85\,GHz band provides a pathway to an optimum and robust performance for ngEHT
in sub-millimeter VLBI, and strongly recommmend its inclusion in the
simultaneous multi-frequency receiver design.
