When you setup a Cisco or Grandstream ATA to simulate a SIP Trunk as a PBX account, you must use the extension Redirection on Busy to have it rollover to the next line. When you do this, the roll over call can take up to a minute before it rings the second port/line. This happens because by default and extension line can support multiple calls which means the first call is not really busy but actually a call waiting situation. The ATA is sending a call waiting signal that no one hears for 30 to 60 seconds, then it goes busy and rings the second port immediately.
This setting has to be overridden in the actual PBX server ad requires Talking Platforms technical support.
Please note this applies to however many ATA's you setup in rollover from port to port and ATA to ATA. So you could have 5 ATA's with each one rolling over to the other and requires this change to avoid the delay.