Does the phone attached on the microwave need network to send D-mails in the past in Steins Gate?
I think the D-mail actually alters the timeline into the one that has received the message. It doesn't send at that present time but it is sent in the past. 

The sending of the message actually becomes the change between the timelines.

But this makes me wonder how the pager thing worked? Different timelines, different technologies. I mean what was the sender number in that message received by the pager? Is that really possible?

Please correct me if I'm wrong.

EDIT : (after OP comment)
The answer is NO,the D-mail does not require network to be sent.