It looks like your answering machine has some difficulty to recognize the CPC (calling party control) sent by OBi device. When OBi sees a call is ended, it sends out a CPC, then the dial tone for next new call... so, you may adjust CPC setting to make it suitable for your answering machine --
go to: Physical interface --> PHONE port --> CPCDuration & CPCDelayTime
You may make CPC duration a little longer than 500 ms, say 800 ms, see how it goes.. Also, adjust CPCDelayTime to be a little shorter, < 1000 ms... Just do a few experiments and find a good combination for your answering machine ...