Is there a way to send and receive SMS when a cellular signal is not available, but WiFi is? I'm often on-site at locations where the building has a strong WiFi (Internet) connection, but a simple cell signal is spotty at best.
Several services exist to send SMS via the Internet. One of the most popular (an possibly most reliable) is Google Voice.
SMS is tied in with GSM telephony standard, so the answer to your question is No.
But what you can do, is use Viber, Whatsapp to send something similar to SMS (not the GSM variety!) to each party provided they have the appropriate app installed.
Use smtp to send as email thru sms email gateway. fairly easy to do. Gmail users can send 500 emails per day. You can leave the subject blank and send it to the gateway address which is usually something like email@example.com. vtext.com is for text sms only for verizon, also seems to handle many metropcs and walmart numbers. works well for simple text messages. I do it all the time from my email account when i need to text someone and im out of minutes.
refer to the sms gateway list on wikipedia. it really is rediculously simple, the issue is knowing which gateway to which number. typically, users would select their carrier when submitting their number to you say via website. used to see this alot.
the code u need is here... https://stackoverflow.com/questions/25136025/sending-mail-in-android-without-intents-using-smtp