From CME Admin Guide
http://www.cisco.com/en/US/partner/docs/voice_ip_comm/cucme/admin/configuration/guide/cmecover.html#wp1145800
If call-waiting is enabled (the default), parallel hunt groups support multiple calls up to the limit of call-waiting calls supported by a particular SIP phone model. You may not want to use unlimited call-waiting, however, with parallel hunt-groups if agents do not want a large number of waiting calls when they are already handling a call.
If call waiting is disabled, parallel hunt groups support only one call at a time in the ringing state. After a call is answered (by one of the phones in the hunt group), a second call is allowed. The second and subsequent calls ring only the idle phones in the hunt group, and bypass the busy phone that answered the first call (because this phone is connected to the first call). After the second call is answered, a third call is allowed, and so on until all the phones in the parallel hunt group are busy. The hunt group does not accept further calls until at least one phone returns to the idle/on-hook state.
When two or more phones within the same parallel hunt group attempt to answer the same call, only one phone can connect to the call. Phones that fail to connect must return to the on-hook state before they can receive subsequent calls. Calls that arrive before a phone is placed on-hook are not presented to the phone. For example, if a second call arrives after Phone 1 has answered the original call, but before Phone 2 goes back on-hook, the second call bypasses Phone 2 (because it is offhook).
When a phone returns to the idle/on-hook state, it does not automatically re-synchronize to the next call waiting to be answered. For example, in the previous scenario, if the second call is still ringing Phone 3 when Phone 2 goes on-hook, Phone 2 does not ring because it was offhook when the second call arrived.
HTH,
Chris
