Not using GNU Radio but perhaps you might be interested in http://www.whence.com/minimodem/ http://www.whence.com/minimodem/. It only supports really old stand

Not using GNU Radio but perhaps you might be interested in http://www.whence.com/minimodem/ http://www.whence.com/minimodem/. It only supports really old standards, up to 1200bps. But later modems should, in theory, be backward compatible so if you don't care about speed this could possibly work.