As it is Nortel I am assuming that this is IS-95, i.e. classic CDMA. Check that the Radio stack of note4 aligns with the Radio Network of Nortel/Ericsson. aligns.
A few rounds of Interworking tests will be required to determine the Root-Case Analysis. Check that the Nortel / Ericsson Radio Network has all of the latest patches deployed.
VBR/ Wallis Dudhnath