I am working on the device from which I want to send SMS/Call Alerts using GSM, Alerts are pre-recorded. In case the GSM fails I want my system to call using PSTN Line and play pre-recorded messages.
I am new to Asterisk. Can anyone please let me know what hardware will be required?
I will be running Asterisk on Raspberry Pi.
Accessing the GSM network requires a GSM dongle (USB) or GSM gateway (SIP).
Accessing the PSTN network requires PSTN hardware like a dongle (USB), ATA (SIP), channel bank (USB, SIP, T1), etc.
There are commercial service providers (Twillio, Lime, Vitelity, …) that can provide access to these networks obviating the need for hardware.
Thank you sedwards, I will search these hardware addons and will get back to you here if need help.
You may want to approach this from the other side 
You can sign up with a provider, kick in $20 or so, and develop a ‘proof of concept’ or working model.
You will refine your needs and expectations without spending a lot of ‘hard cash.’
Then, if your volume justifies the expense, dive it.
I am already using Twilio and just wanted to know if this is possible or not in case my user base increases.