You should read carefully my posts

There are two ways to configure No-Answer - at application level or using Extension keys. Configuration on application level is global = valid for all agents. Configuration using Extension keys is valid just for a call having these keys attached to it.
So I would recommend you to do following:
- Configure the options 'agent-no-answer-action', 'agent-no-overflow' and 'agent-no-answer-timeout' in SIP Server application's Options using the most common values e.g. timeout = 10 seconds
- For special call types when you need to extend the ringing time to 20 seconds attach the keys listed below to Extensions collection in the routing strategy before targeting the agents
ExtensionUpdate['NO_ANSWER_TIMEOUT','20']
ExtensionUpdate['NO_ANSWER_OVERFLOW','
<value>']
ExtensionUpdate['NO_ANSWER_ACTION','
<value>']
R.