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For non-ACD (queue) related calls, simply forwarding an extension to an off-network phone is all you need do. This can be done by following the steps outlined in the article How do I forward all my calls?.
PBXtra supports off-network phones in queues by enabling the "Queue Press-to-accept" option on an extension or virtual extension that is forwarded to an off-network phone. A virtual extension is created by adding a new extension and selecting 'Virtual Extension' option under "Phone / Devices". The "Queue Press-to-accept" option is a PBXtra Call Center Edition feature that forces a user (agent) to press '1' after picking up a queue call to actually accept it. If this option is set to 'no' (default), then the call is answered as soon as the agent picks up the handset. The call will also be answered if voice mail or an answering machine picks up.
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