The reason why delivery receipts are not supported for a network or a country can vary. It can also be possible that the network doesn't support handset delivery receipt, but the network sends an acknowledgement when they receive the message.
Due to this, the response for messages to one of these networks can be different.
For Example :
In the US, most networks don't support handset delivery receipts when messages come from non registered short codes or if they come from long codes. In this case, you will receive a status : "ACCEPTED". This means that the network has accepted the message and will attempt to deliver it.
In the Philippines, the networks can sometimes send fake delivery receipts depending on the type of traffic or not send delivery receipts at all. Due to this, for messages sent to the Philippines, we wait 72 hours and then we send a final status of "EXPIRED"