ISPs do not use the same format as the same-numbered OSPs. ISP opcode 04 is not Unit to Unit Voice Grant, it is Unit to Unit Voice Request, and there is no channel number specified.
Octet 2 of UU_V_REQ is Service Options, which we've discussed before in another thread. 04 = non-emergency, non-protected, half-duplex, circuit mode, priority 4 (default priority).
Furthermore a channel grant for a unit-to-unit voice request, which would happen on the OSP side were it to be granted, would not be on a "simplex" channel, it would be on one of the trunk frequencies; it's an I-Call in other terms.