I have some problems with the Avaya AES. I need to know if transferring a call to an external number is possible?

Scenario looks like this:
1. Client call to the agent.
2. Agent answered the call.
3. Agent tries to transferring call to an external number.

I am using Avaya OneX Agent.
Avaya Application Enablement Services
SW Version: 8.1.2.1.0.6-0

Yes it is possible.
Are you seeing a error code? If so what?
What transfer methodology are you using (blind/consultative)?
What specific Method are you using in the API?
Are you sure the destination number is dialable from the agent (have you mimicked the CTI functionality at the specific extension you are working with manually and verified there isn't something else going on)?
There is a flag in Communication Manager to allow/prevent trunk to trunk transfers, does it work if a internal station calls the agent?
Do both trunks provide disconnect supervision - there is another check to disallow a trunk to trunk connection where disconnect supervision is not provided.

Thank you for answer.
I am using DMCC my request looks like this:

{"_heldCall":{"_deviceID":{"_content":"1011024:SWLINK1::0","_typeOfNumber":{"type":17,"stringValue":"other"},"_mediaClassList":["notKnown"],"_bitRate":{"type":0,"stringValue":"constant"},"id":{}},"_callID":"1912","id":{}},"_activeCall":{"_deviceID":{"_content":"1011024:SWLINK1:10.245.16.52:0","_typeOfNumber":{"type":0,"stringValue":"dialingNumber"},"_mediaClassList":[],"_bitRate":{"type":0,"stringValue":"constant"},"id":{}},"_callID":"1913","id":{}},"id":{}}

While I am sending it by using transferCall method - TransferCallResponse response = callControlSvcs.transferCall(request);

I am getting:
ch.ecma.csta.errors.ConferenceMemberLimitExceededException: null
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_275]
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_275]
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_275]
at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_275]
at java.lang.Class.newInstance(Class.java:442) ~[na:1.8.0_275]
at com.avaya.mvcs.proxy.ClientNegAckToException.convertErrorToException(ClientNegAckToException.java:130) ~[tcc-3.3.3-p11-fat.jar:na]
at com.avaya.mvcs.proxy.XmlGatewayClient.handleRequest(XmlGatewayClient.java:250) ~[tcc-3.3.3-p11-fat.jar:na]
at com.avaya.mvcs.proxy.Pipeline.handleRequest(Pipeline.java:346) ~[tcc-3.3.3-p11-fat.jar:na]
at com.avaya.mvcs.proxy.ClientProxy.routeRequest(ClientProxy.java:699) ~[tcc-3.3.3-p11-fat.jar:na]
at com.avaya.mvap.svcproxy.ServiceProxy.marshallRequest(ServiceProxy.java:131) ~[tcc-3.3.3-p11-fat.jar:na]
at com.avaya.mvap.svcproxy.CallControlServicesProxy.transferCall(CallControlServicesProxy.java:394) ~[tcc-3.3.3-p11-fat.jar:na]
at com.tenfold.tcc.phonesystem.avaya.AvayaAdapter.consultTransfer(AvayaAdapter.java:938)

The type of error is: "System resource availability error, reason: conferenceMemberLimitExceeded, type: 25"

what does it mean?
I am using OneX Communicator

Communication Manager has a limit of 6 parties in a conference (Unless you have set a system setting in release 8 that allows 12 party conferences).

If you run a status station command on Communication Manager SAT you can see how many parties are in the active call on a station.

Thank you.

I transferring external call to an external number. So is there any other way, when this exception can be thrown (if I have only 3 active parties)?

Please open a technical support request and provide a full trace of your use case after enabling AES Finest level logging.

https://www.devconnectprogram.com/site/global/products_resources/avaya_aura_application_enablement_services/support/faq/dmcc/other.gsp#60

You did not respond to any of my statements regarding checking out Communication Manager's willingness to allow the transfer, thus since I assume Communication Manager is what is blocking your transfer attempt, verify that you can invoke the transfer from the actual station you are using in your test (from the phone itself). If you can not do the transfer on Communication Manager, you won't be able to do it via CTI.