Best answer by Dinh
Are you an existing customer and You received the phone/SIM from an online order? what phone was it? when you pop your old SIM in, make sure your old SIM was put into the correct tray.
If you want to use the new SIM, you need to activate the new SIM before using it. Transfer your old SIM into new SIM following the instruction here https://www.koodomobile.com/help/koodo-sim-support