not evident to give you a response ...
what about your Software reliability ?
and environnement ?
what about the Power supply, EMC distubations... etc ..
The SIM900A could be enough reliable itself (MTBF,technical caracteristiques,firmware test,
but depend of the usage context .
i France ,we have a lot of devices coming fro RPC with the mark "CE" compliant ..
but this CE mark is not validate when using it in another context .
Device + Other hardware + Application software ..must be "CE" certified by doing many tests.