Добавляем в /boot/loader.conf
umodem_load="YES
u3g_load="YES"

И грузим в ручную
kldload u3g && kldload umodem
Приводим /etc/ppp/ppp.conf к такому виду (оператор Мегафон)

default:
set log Phase Chat LCP IPCP CCP tun command
ident user-ppp VERSION
megafon3g:
set device /dev/ttyU0.0
set speed 460800
set dial "\"\" \
AT OK \
AT OK \
ATS0=0 OK \
AT OK \
AT&FE0V1X1&D2&C1s0=0 OK \
AT OK \
AT+CGDCONT=1,\\\"IP\\\",\\\"internet\\\" OK \
ATDT*99***1# CONNECT"
set login
set authname "megafon"
set authkey "megafon"
disable ipv6cp
disable pap
disable chap
enable dns
set ifaddr 10.0.0.1/0 10.0.0.2/0 255.255.255.0 0.0.0.0
add default HISADDR

И пробуем
ppp -ddial megafon3g
В принципе все )))