After i bought my N900 i bought also N810,I love maemo platform but n810 can't do calls. Until now !
I bought Telit GE865 gprs module ( i know, i know, gprs only but i wait a new module ) and make pigs fly.
Telit module can do calls with no more than a earpiece, mike, two resistors & capacitors, a battery and an interface with N810.
It has a lot of functionality like fax, tcp stack ant python script interpreter. http://www.telit.com/en/products/gsm...p_ac=show&p=47
Possible solutions of module integration with N810 :
1 stand alone phone with limited conection with 810 only for dialing and sms
2 make full implementation like data connections & usb sd card increased size storage

Module can be hiden under the 810 hood replacing mini sd card slot wich is almost useless.
I do some hardware hack to PL2303 usb-serial bridge cause telit module have 2.8 volt serial interface.