Yes, by means of sharing the same Apple ID.

You can set up and use the same Apple ID for the 2ndLine app on the representative's phone, you can use the 2ndLine app by retaining your purchased number on the delegated phone. The 2ndLine number and subscription work on Apple ID used on the device.

Please be aware that you can use the 2ndLine app on only one device even if you are using a common (same) Apple ID on multiple iPhones/iPads.