Can this happen?

[txwebman]

Was the original call a voice call or strictly a hang up when you answered. If there was no voice, sounds like something simpler like a trojan or some kinda remote access that someone has compromised the system with. Using your own modem to dial your cel #. Is your cel # on your computer? Who knows your cel #? Anyone one you might know that could be after a little revenge or mischevious acts on you?

[The3ntropy]

Korndogma,
One other possibility that it could be. Remember people are usually dumb, aswell as someone who would play such a childish prank on someone like that. An extremely easy way to place, intercept, and tap calls going to and coming from a house is just to run a line, or a remote divice can be attached to, your little gray telephone box that is outside. 99% of the time there is just a clip that is keeping it shut but if someone can gain access to that little gray box, they can then easily place a call from any one of the lines in your house, or apartment complex for that matter. {The set up to do so is aswell easy, there are 4 different colored wires, there are 4 wires in a telephone cord, Hmmm}