Are replies FREE of charge?

Yes, you will not be charged for incoming text messages (SMS), they cost 0 credits.  There is a charge for inbound MMS messages, it is usually 1 credit for USA and most countries.  For a complete list of MMS inbound credits charge go here.

Incoming SMS alerts: you can set text message forwarding for ALL inbound SMS or MMS, and you will be able to receive the text messaging alerts on your mobile phone or email.