modem/phone mass sms concept

Please i would like someone to explain to mean exactly how mass sms softwares that uses phones/utms modem works. do they use internet access or do they just take out of my airtime like regular sms through phone or is it cheaper. if the softwares take away credit airtime just like regular sms through phone, then why use them since we can send mass sms through our phone but harder though. talking about softwares like smscaster that uses phone/modem. I'm Confused

bump any help......

i think that can bring cops to your door