It should be used pin RI (RING Indicator).
Pin from Ublox is connected to the microcontroller. It can be seen from the scheme.
But most likely it is able to receive and messages without using RI. The official Arduino GSM Shield does not use RI. It is based on Quectel M10. Also some Shield based on SIM900 do not use RI. Can most likely to perform the software without using an external interruption. But however the pin RI is already connected 
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