Hi, yea, I know one:
GSM-Gateways: ECOTEL
It's a product of some German manufacturer. Have been talking to people who had some really good experiences with this thing: Ecotel VoIP, Vierling VoIP GSM Gateway. Most other manufactureres have quality problems.
The device is based on SIP and has 2, 4 or 8 GSM channels.
Regards
Rich