The vty port on a 520 are defined as "limited" function ports by IBM. That is, they are designed to be typically used only for a dumb console, or a dial in modem. BUT, for the modem you do not have full modem control, cannot set
RTS/CTS, and will only reliably support only the 19,200 baud rate. If you must support a dial in modem it is suggested you add a "5723 2 PORT ASYN.EIA-232 PCI ADPT" which has a list of $285. It is easy to install and configure, and will solve your problem as it did for me.