You are not supposed to dial 99# from the phone UI. It's special number that gets interpreted by the phone hardware itself and switches it to essentially router

You are not supposed to dial 99# from the phone UI. It's special number that gets interpreted by the phone hardware itself and switches it to essentially router/access-server mode (the 1 at the end is optional parameter which is index of configuration you want, with the configurations being defined by AT+CGDCONT), most common thing that happens is that phone estabilishes PPP session with whatever device that sent ATD*99# and then routes it through packed based celular networkg (ie. GPRS/EDGE/UMTS/LTE), the mechanism is not specific to IP (although the actual network-side transport is always over IP) and other L2 protocols than PPP are sometimes supported. There is no reason why smartphone has to use Hayes protocol to communicate with baseband (and thus this mechanism), but many do.