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.
| M590E |  |
| SIM800C |  |
| A6 |  |
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”.