Your thinking about it the wrong way. It has nothing to do with what you are using the call for.
What Digium sells is the g729 codec with a certain number of licenses. The only time that a license is used is when transcoding is required.
A call has two legs to it, or sides. If both of the sides are not g729, and only one side is g729, then transcoding and a license is required. Period.
Basically, transcoding is required anytime both sides of the call are not using the same codec.
If you want to use g729 with no transcoding make absolutely sure that every SIP device you connect has deny=all and allow=g729. Also make sure that your VOIP provider allows g729 and force it to use that only. If you are using any analog lines connected to Asterisk via whatever hardware you are using, make sure that is coming in as g729 too.
Ohhhh, and you will need to configure voicemail, sounds, moh, etc. Everything in the system must be set to g729 if you want to create a system with no transcoding.