We have two different types of GSM modules & shields under the FONA brand name. The FONA 800 (GSM/GPRS)

and the FONA 808 (GSM/GPRS + GPS too)

Here's the key similarities

• Both have a 2G GSM/GPRS cellular core
• Both can do voice, data, text
• Both require a LiPoly battery and GSM antenna
• Both can use a 3.5mm headset
• Both recharge over the microUSB jack
• Both have a buzzer driver

Here's key differences

• Size: the SIM808 based FONA breakout is larger. Shields are the same size
• The 808 has a GPS module integrated as well
• The 800 has external 8 ohm speaker driver (external audio) whereas the 808 has only headset audio and 32 ohm speaker driver
• The 800 has an FM tuner for listening to radio