I notice this new package called nohands, that is supposed to make the N900 acting as a HFP bluetooth headset. That would be a great option IMO!
After installing, I launched the hfpd daemon in the terminal:

N900:~$ hfpd -f
D-Bus: connected
D-Bus: Exported "/org/freedesktop/DBus/Local"
D-Bus: Exported "/net/sf/nohands/hfpd/soundio"
D-Bus: Exported "/net/sf/nohands/hfpd"
Unsuitable SCO MTU values 64:1 detected
To fix this, run, as superuser, "hciconfig hci0 scomtu 64:8"
^C
N900:~$ root


BusyBox v1.10.2 (Debian 3:1.10.2.legal-1osso30+0m5) built-in shell (ash)
Enter 'help' for a list of built-in commands.

N900:~# hciconfig hci0 scomtu 64:8
N900:~$ hfpd -f
D-Bus: connected
D-Bus: Exported "/org/freedesktop/DBus/Local"
D-Bus: Exported "/net/sf/nohands/hfpd/soundio"
D-Bus: Exported "/net/sf/nohands/hfpd"
SCO MTU: 64:8 Voice: 0x0060
Bluetooth System Started
*** Your configured device class may not be recognized as a hands-free
*** Edit /etc/bluetooth/hcid.conf and change:
*** class 0x780408;
HCI Command status: 0x00 0x01 0x0409
HCI Command status: 0x00 0x01 0x041b
HCI Command status: 0x00 0x01 0x0419
HCI Name request complete (0): "00:08:F4:20:63:AF" -> "ulisse"
HCI Command status: 0x00 0x01 0x0409
HCI Command status: 0x00 0x01 0x041b
HCI Command status: 0x00 0x01 0x0419
HCI Name request complete (0): "00:08:F4:20:63:AF" -> "ulisse"
Creating record for BDADDR 00:08:F4:20:63:AF
AG 00:08:F4:20:63:AF: Refusing connection
Destroying record for 00:08:F4:20:63:AF

The device is recognized as headset, but at the end it does'n connect. I have no etc/bluetooth/hcid.conf file at all, can be the issue related to that?