This problem is observed because the line runs out of virtual lines. A line can have only two virtual lines active at the same time. When a call is on hold or parked, another line is required to transfer or park subsequent calls. Since the line is occupied by the first call, the transfer and park options will not show up.
To avoid this situation, have another internal normal Dialed Number (DN) assigned to one of the lines of the agent's IP phone. Have the agent perform the following steps:
Press Transfer.
Enter the number of your internal line, not the ICD line.
Press Transfer again before it connects.
Answer the ringing line. Press the Line button or answer.
Press Park.