Just like the Softphone, Groundwire supports having multiple SIP Accounts registered for incoming calls. But Groundwire also supports multiple lines. You can continue to receive new calls while on a call (call waiting). Or even place a call on hold and make additional calls. If your provider or PBX supports it, you can even make multiple calls from the same account.
If you are on a call and receive a second call, you will hear a short tone. Just tap on answer to accept it or reject to decline it. If you accept it, your other call will be put on hold automatically. To go back to the original call, tap on the details of the current call at the top of the screen. That will bring up a list of the currently active calls. Tap on the call you want to go back to, then tap on hold to take it off hold and resume the call. There is no need to place the current call on hold first, taking one call off hold will atomatically place the other one on hold.
To make additional calls without disconnecting your current call, do the following. Tap on the Home button on the in call screen. You can then switch SIP accounts if you need to. You can dial directly from the keypad or from Quickdial, History or Contacts. As soon as you dial the number, the original call will be placed on hold. You can then go back to the original call whenever you like.
Note: There is no built in limit to how many calls Groundwire can handle this way. But it is effected by the limitations of the device and the network. For example, an older iPhone with a 3G connection would likely have issues with more than two calls, but a recent iPhone with a strong, fast Wi-Fi connection could likely handle five calls with no problem.