As of iOS10, this article has been updated.
For most users, Push will be the way to go. Unless your provider has some restriction about where you can register from or you have a PBX that is only available from the local network, Push will almost always be the best option for incoming calls.
But if you do have some need to avoid using Push, this is how to configure multitasking.
From the keypad, tap on settings, then incoming calls. Set to Standard.
Then go to the settings of the SIP Account. Make sure incoming calls there is set to Use Global Setting. You can set incoming calls at the account level. but once you have one account that is using Standard, there is no great benefit if the others continue to use Push. Basically, if even one account needs to be set to Standard, most of the cost to battery has already occurred, using Standard for any additional accounts probably won't effect the battery life much.
keywords: multitask, background, acrobits, softphone, groundwire, receive