kblim,

there are so many GSM-Modules (Engines) or even "standalone" GSM-Terminals available at reasonable prices.

OK, for a single shot prototype for home use one could try a mobile phone, but for a commercial product?

If you require just one or two GSMs for a hobby project go get you some used phones with a serial port. You can get used phones at very low prices.

However, I would recommend a GSM-Module or Terminal.

A good choice is SIEMENS or TELIT.
SIEMENS TC35i is a robust low cost GSM Terminal.
The TELIT Terminals offer some nice extra features but are more expensive.

The Telit PYTHON Terminal does even have some GPIOs, plenty of memory and supports I2C, so for many applications you wouldn't need an additional Micro at all!

This will reduce overall development cost and time to market to an absolute minimum.