Siemens I-SURF - Manual (Page 56)

10. Internet access
50
For example, if the connection uses ("Using:") ISDN Line 0, you may add ISDN
Line 1 to Set additional devices to activate the multilink function for this
connection. This will make 128 kbps bandwidth available to you.
You may add another connection by clicking Add. You need to enter the channel
not yet used for this connection.
Any multilink connection, however, requires that your dial-up server supports this
feature and that your account has the necessary access privileges. Multilink
connections, for instance, do not work for CompuServe.