Looks like the GSM module will be added to the Ethernet Module. Found several modules in price from 1.27 to 4 USD. Basically UART operated with support of GSM + GPRS, TCP/IP, UDP/IP, SMS etc. The cheapest kit is data only, but some of the others include voice as well. More important is that datasheets and programming guides are available.
The M590E is SMS/Data only and cost 1.27 USD, the others include voice and cost 3-4 USD. More expensive modules with 3G/4G etc exist, but GSM/GPRS should be sufficient. GPRS have a bandwidth of ca 85Kbps.
I need to find space for these modules + SIM Card and antenna, but I think that is very doable. It is also an option to use one of the breakout boards “as is”.