Just adding to this thread in case this information is helpful for anyone out there. I faced the same problem, the issue is the RingTimeout in the Physical Interfaces - Line settings. Try setting it to max, 1920ms. This is still not enough for my enterphone system, but at least it gave me 1.92 seconds to answer, which resulted in occasional success letting people in.
If the max isn't enough for you, get another ATA that permits a longer RingTimeout setting. SPA232d, Grandstream HT813 are examples.
Update: the SPA232d has more than one parameter named RingTimeout. The advertised one goes for as many seconds as you'd like. The "international settings" one that affects this issue has the same limits as the OBI110. Trying the audio codes mp 114, which seems to allow FXOBetweenRingTime of many seconds.