The Voice Modem is not SIP, you can't use it as a trunk in 3CX. An ATA or Gateway (The majority) are SIP compliant and convert the voice to digital using standard Codecs. They also handle all of the SIP messaging involved. And, yes, gateways are more expensive than an ATA. But if you have to feed the line out from anything, including Magicjack, PSTN, a Vonage ATA,etc., and you want to "bring it into" a SIP PBX as a trunk, they you will need a Gateway. Magicjack has had a while now to figure out how to "deal" with customers trying to use other devices with their service. I'm sure that there have been several "upgrades" over the last year to plug any loopholes that people have come up with, especially if they are made public on the net.