Is it permissible to marry without divorcing someone to whom one was married over the phone with unknown witnesses? And what is the ruling on telephonic sexual intimacy in this case?

What occurred between you over the phone is not considered a valid marriage (zawaj shar'i) due to the absence of the conditions for its صحت (soundness). Therefore, you owe nothing to this man, and you are able to marry someone else. Scholars have differed on the necessity of divorce (talaq) or annulment (faskh) in an invalid marriage (zawaj fasid). Some, like Ash-Shafi'i, hold the view that it is not necessary. The expiation for the unlawful enjoyment that occurred between you over the phone is repentance to Allah by abandoning the sin, regretting it, and resolving not to return to it.