iSip on iPhone (Version 4.7 and higher) has been verified can work successfully with MagicJack account.
Plus, now you can receive 'PUSH'ed incoming call when iSip is not running if you turn on the 'PUSH' feature.
Below are some screenshots showing how to config magicjack account on iSip:
Account's basic setting:

Account's advanced asseting:

Turn on PUSH for the account.
Tap on the capital P can bring you to this screen. Just turn the switch on.
Make sure iPhone's 'notification' is also turned on:

When PUSH is turned on, the capital P turns into dark green:

STUN setting:

Hope you can enjoy the app!