GPRS or Edge Howto?

The ESP8266 problem is because of their stupid use of Linked/Unlinked with no connection number. The SIM900 shouldn't have that.

I guess the problem here is that at.isBusy() is reporting false because it's not actually busy, just waiting for >.

Why not add your own variable to track whether you're waiting or not?

send: function(sckt, data) {
    if (at.isBusy() || this.sendWaiting || socks[sckt]=="Wait") return 0;
    if (!socks[sckt]) return -1; // error - close it
    this.sendWaiting = true;
    var f = function(d) {
      at.unregister('>');
      this.sendWaiting = false;
      if (d != sckt + ", SEND OK") return; // <------- this doesn't seem to do anything?
    };
    at.register('>', function() {
      at.write(data);
      return "";
    });
    at.cmd('AT+CIPSEND='+sckt+','+data.lengt­h+'\r\n', 10000, f);
    return data.length;
  }