For CTI Server interface, the "40 byte call variables" actually means 39 character bytes and a null terminator. This is contrasted with the ICM VRU interface where a 40 byte call variable means 40 bytes plus terminator.

Beginning with ICM version 7.1(1) and later, defect CSCsb61534 was fixed to allow CTI Server to handle 40 bytes plus null terminator. CTIOS has not yet been updated with this fix and the C-API will not handle 40 bytes plus null terminator. Only those applications writing their own socket level messaging will be able to handle 40 bytes plus null terminator. If you specify CTI Server protocol version 8 or earlier, only 39 bytes plus null terminator will be used. There is an Engineering Special ES28 available against 7.1(3) to fix an issue where 40 bytes plus null terminator is delivered causing the CAPI not to function correctly.