plug phone into USB:

Nov  3 09:18:37 moon kernel: [ 1536.812078] usb 3-1: new full speed USB device using uhci_hcd and address 2
Nov  3 09:18:38 moon kernel: [ 1537.020830] usb 3-1: configuration #1 chosen from 1 choice
Nov  3 09:18:39 moon kernel: [ 1538.254800] cdc_acm 3-1:1.8: ttyACM0: USB ACM device
Nov  3 09:18:39 moon kernel: [ 1538.257881] usbcore: registered new interface driver cdc_acm
Nov  3 09:18:39 moon kernel: [ 1538.259081] cdc_acm: v0.26:USB Abstract Control Model driver for USB modems and ISDN adapters
Nov  3 09:18:39 moon kernel: [ 1538.372826] usbcore: registered new interface driver cdc_ether
Nov  3 09:18:39 moon kernel: [ 1538.397779] usb 3-1: bad CDC descriptors
Nov  3 09:18:39 moon kernel: [ 1538.398675] usbcore: registered new interface driver rndis_host
Nov  3 09:18:39 moon kernel: [ 1538.442686] usb 3-1: bad CDC descriptors
Nov  3 09:18:39 moon kernel: [ 1538.443600] usbcore: registered new interface driver rndis_wlan
Nov  3 09:18:39 moon nm-system-settings:    SCPlugin-Ifupdown: devices added (udi: /org/freedesktop/Hal/devices/usb_device_421_2f_noserial_if8_serial_unknown_0, iface: (null))
Nov  3 09:18:39 moon NetworkManager: <info>  ttyACM0: driver is 'cdc_acm'.
Nov  3 09:18:39 moon NetworkManager: <debug> [1225671519.769338] setup_monitor_device(): No monitoring udi provided
Nov  3 09:18:39 moon NetworkManager: <info>  Found new Modem device 'ttyACM0'.
Nov  3 09:18:39 moon NetworkManager: <info>  (ttyACM0): exported as /org/freedesktop/Hal/devices/usb_device_421_2f_noserial_if8_serial_unknown_0
Nov  3 09:18:43 moon NetworkManager: <info>  (ttyACM0): device state change: 1 -> 2
Nov  3 09:18:43 moon NetworkManager: <info>  (ttyACM0): deactivating device (reason: 2).
Nov  3 09:18:43 moon NetworkManager: nm_system_device_flush_ip4_routes_with_iface: assertion `iface_idx >= 0' failed
Nov  3 09:18:43 moon NetworkManager: nm_system_device_flush_ip4_addresses_with_iface: assertion `iface_idx >= 0' failed
Nov  3 09:18:43 moon NetworkManager: <info>  (ttyACM0): device state change: 2 -> 3

Now I can run 'pon three' for pppd, and get 3G internet.
But try NetworkManager GUI:

Nov  3 09:22:23 moon NetworkManager: <info>  Activation (ttyACM0) starting connection 'Auto Mobile Broadband (GSM) connection' 
Nov  3 09:22:23 moon NetworkManager: <info>  (ttyACM0): device state change: 3 -> 4 
Nov  3 09:22:23 moon NetworkManager: <info>  Activation (ttyACM0) Stage 1 of 5 (Device Prepare) scheduled... 
Nov  3 09:22:23 moon NetworkManager: <info>  Activation (ttyACM0) Stage 1 of 5 (Device Prepare) started... 
Nov  3 09:22:23 moon NetworkManager: <debug> [1225671743.339641] nm_serial_device_open(): (ttyACM0) opening device... 
Nov  3 09:22:23 moon NetworkManager: <info>  Activation (ttyACM0) Stage 1 of 5 (Device Prepare) complete. 
Nov  3 09:22:23 moon NetworkManager: <info>  (ttyACM0): powering up... 
Nov  3 09:22:23 moon NetworkManager: <info>  Registered on Home network 
Nov  3 09:22:23 moon NetworkManager: <info>  Associated with network: +COPS: 0,2,"50506",2 
Nov  3 09:22:24 moon NetworkManager: <info>  Connected, Woo! 
Nov  3 09:22:24 moon NetworkManager: <info>  Activation (ttyACM0) Stage 2 of 5 (Device Configure) scheduled... 
Nov  3 09:22:24 moon NetworkManager: <info>  Activation (ttyACM0) Stage 2 of 5 (Device Configure) starting... 
Nov  3 09:22:24 moon NetworkManager: <info>  (ttyACM0): device state change: 4 -> 5 
Nov  3 09:22:25 moon NetworkManager: <info>  Starting pppd connection 
Nov  3 09:22:25 moon NetworkManager: <debug> [1225671745.099777] nm_ppp_manager_start(): Command line: /usr/sbin/pppd nodetach lock ttyACM0 noipdefault usepeerdns ipparam /org/freedesktop/NetworkManager/PPP/0 plugin /usr/lib/pppd/2.4.4/nm-pppd-plugin.so 
Nov  3 09:22:25 moon NetworkManager: <debug> [1225671745.219808] nm_ppp_manager_start(): ppp started with pid 6383 
Nov  3 09:22:25 moon NetworkManager: <info>  Activation (ttyACM0) Stage 2 of 5 (Device Configure) complete. 
Nov  3 09:22:25 moon pppd[6383]: Plugin /usr/lib/pppd/2.4.4/nm-pppd-plugin.so loaded.
Nov  3 09:22:25 moon kernel: [ 1764.456896] PPP generic driver version 2.4.2
Nov  3 09:22:25 moon pppd[6383]: pppd 2.4.4 started by root, uid 0
Nov  3 09:22:25 moon pppd[6383]: Using interface ppp0
Nov  3 09:22:25 moon pppd[6383]: Connect: ppp0 <--> /dev/ttyACM0
Nov  3 09:22:25 moon NetworkManager: <info>  (ttyACM0): device state change: 5 -> 6 
Nov  3 09:22:25 moon pppd[6383]: PAP authentication succeeded
Nov  3 09:22:25 moon NetworkManager: <info>  (ttyACM0): device state change: 6 -> 7 
Nov  3 09:22:25 moon kernel: [ 1764.764989] PPP BSD Compression module registered
Nov  3 09:22:25 moon kernel: [ 1764.904183] PPP Deflate Compression module registered
Nov  3 09:22:26 moon pppd[6383]: LCP terminated by peer


Now the laptop has hung completely.