/dev/cuaU0.0みたいなデバイスが出現しませんか？

あれば、/etc/ppp.confに以下を書いて

emobile:
set device /dev/cuaU0.0
set speed 115200
set dial "ABORT BUSY ABORT NO\\sCARRIER TIMEOUT 5 \
\"\" AT OK-AT-OK ATE1Q0 OK \\dATDT\\T TIMEOUT 40 CONNECT"
set timeout 180
set phone "*99***1#"
set authname "em"
set authkey "em"
set ifaddr 10.0.0.1/0 10.0.0.2/0 255.255.255.0 0.0.0.0
delete ALL
add default HISADDR
enable dns

これで接続できる
ppp -background emobile