The one or both speakers could use a handshake noise at the start of the call to tell the receiver that it's capable of "speaking" a modem protocol. It might change a little every time, or be of an especially low or high frequency so that a person doesn't realize they're talking with a computer. After handshaking, the receiver could send a URL that would allow the channel to be upgraded to the Internet... or not. English is a good fallback if both people speak it and you can't find a more efficient channel.