If you've spotted High-Level talking about the "LC Phone" offering, this is a paid offering that High-Level has recently launched.
Under the hood, it's simply a Twilio service with re-billing features so an agency (Growably) can re-sell VoIP / Telephony to their clients to make more money.
At the moment, we don't have any short-term plans to offer the LC Phone system.
Which means that if you would like to integrate a Phone / VoIP / Telephony provider into Growably, you'll need to use your own Twilio account.
You still get all the same features, they'll just show up slightly differently.