You can have several calls in progress at the same time. That includes calls that are alerting you and calls that are on hold. However, you can only have one connected call at any time. Each call is shown by a tab on the left and the icon will indicate the state of that call (connected, held, alerting, etc).
To view the details of any call, simply click on the tab for that call. Just viewing the call details does not answer or otherwise affect the call. To switch to another call, click on the 
call answer or 
retrieve held call button. The call to which you were connected will be automatically put on hold.
Number of Calls
one-X Portal does not limit the number of calls that you can make and receive. While you can only have one call connected at any time, you can have multiple held calls at the same time. However, the phone system limits the number of incoming calls that you can receive.
•Phones With Appearance Buttons
Many Avaya phones have programmable buttons that are configured by the system administrator as appearance buttons. When using one of these phones, each call that you make or receive normally uses an appearance button and can be controlled using that button (press to hold, retrieve, view details, etc).
•For an incoming call, the call is presented on a suitable appearance button if available. If no suitable appearance button is available, the telephone system will treat the call as if you are busy to any further incoming calls.
•For outgoing calls, you can use one-X Portal to make additional outgoing calls even when all your phone's appearance buttons are in use. If you do this, some calls will not be represented by and therefore controllable by an appearance button on the phone.
•Phones Without Appearance Buttons
If the phone you are using does not have appearance buttons, your Calls Waiting On setting, as set by your system administrator, controls the number of calls that you can receive.
•If your Calls Waiting On setting is enabled, you can use one-X Portal to answer an additional incoming call. Once you have two calls being handled, for any additional incoming calls, the phone system treats you as being busy. However, using one-X Portal you can still make additional outgoing calls.
•If your Calls Waiting On setting is not enabled, once you have one connected call to which you are talking the phone system treats you as being busy. However, using one-X Portal you can still make additional outgoing calls.
Notes
1.You can only have one connected call at a time. If you connect to another call (by making a call, answering a call, unpark a call, retrieve a call from hold, etc.), the existing call to which you were connected is automatically put on hold. This is called Auto Hold. Your system administrator can disable Auto Hold for the whole system. If this is done, when you connect to a call, any current call is disconnected.
2.Calls that have been parked are not included. Unlike held calls. they are parked on the phone system rather than held at your phone.