You can change the number of rings the I/O Server will wait before answering the call (using the [Dial]RingCount parameter). If you are using redundant I/O Servers, the primary I/O Server will be called by default. However, by setting [Dial]RingCount to a different value on each of the I/O Servers, you can make the standby I/O Server answer the call if the primary does not.
Consider the following setup:
If you set the ring count to 3 on IOServer1 and 4 on IOServer2, IODev_1 will attempt to call IOServer1. If IOServer1 has not answered the call after 3 rings, IOServer2 will answer it.