You might want to look into what phreakers were doing from the 60's onwards for an idea of how you can stretch the phone network. An old favorite was voicemailb

You might want to look into what phreakers were doing from the 60's onwards for an idea of how you can stretch the phone network. An old favorite was voicemailboxes as a sort of forum. You'd take over some poorly secured company voicemailbox and use it as a communications hub. Of course nowadays the theft aspect is no longer necessary.