For USB modem (tested with multiple Huawei and Mediatek 3G modems, T-Mobile and Vodafone operators):

~ # usb_modeswitch -v 12d1 -p 14fe -m "0x01" -M "55534243123456780000000000000011062000000100000000000000000000" -I
(you will find the correct value in configPack.tar.gz from usb-modeswitch-data or similar in your distribution)

For Bluetooth modems (tested with several Nokia dumbphones):

# bluetoothctl
scan on
pair 00:12:D1:XX:XX:XX
connect 00:12:D1:XX:XX:XX
quit
# rfcomm bind 0 00:12:D1:XX:XX:XX 1 # creates /dev/rfcomm0 device

And then PPP configuration:

~ # cat /etc/ppp/peers/t-cenzor 
/dev/ttyUSB0 OR /dev/rfcomm0
115200
defaultroute
usepeerdns
nodetach
crtscts
lock
noauth
local
debug
nodeflate

connect "/usr/sbin/chat -v -f /etc/chatscripts/t-cenzor-connect"
disconnect "/usr/sbin/chat -v -f /etc/chatscripts/t-cenzor-disconnect"

~ # cat /etc/chatscripts/t-cenzor-connect
TIMEOUT 10
ABORT 'BUSY'
ABORT 'NO ANSWER'
ABORT 'ERROR'

"" 'ATZ'

SAY 'Setting APN FQDN\n'
OK 'AT+CGDCONT=1,"IP","internet"'

ABORT 'NO CARRIER'
SAY 'Dialling!\n'
OK 'ATD*99***1#'
CONNECT ''

~ # pppd call t-cenzor