Push notifications for incoming calls might be delayed on iPod Touches due to sleep mode settings. Disabling Auto-Lock and enabling a persistent Wi-Fi connection on the 4th Generation are potential solutions but both options drain the battery.
When the iPod is active and connected to Wi-Fi, push notifications are received almost instantly. However, if the iPod is in sleep mode, it only checks push notifications every 15 minutes. This leads to potential delays in receiving calls.
To overcome this, you can go to Settings and then set Auto-Lock to Never. Note that disabling Auto-Lock drains the battery rapidly.
The 4th Generation iPod Touch allows a persistent Wi-Fi connection as long as any app has notifications enabled in the iPhone settings. This enables prompt push notifications on the 4th Generation iPod Touch, but it may also impact battery life.