2nd TA with primary number

I don't expect you will be promoting a soft phone at launch.

If you eventually allow credentials for a soft phone with the primary number, people may want to carry a TA or keep a second line available instead of the actual soft phone use. Some may long to have a couple wireless sip phones instead of using the GrandStream. For the purpose of anticipating such possibilities, I just wanted to point out that calling a sip number from a non-Grandstream seems broken.

Perhaps someone using asterisk can confirm the same thing too?

FROM THE GRANDSTREAM:
I can dial 613 to do echo test and all is well.
FROM THE ZyXEL:
I can dial 613 to do echo test and it connects, but gets a busy signal after 18 seconds.

Regular numbers stay connected on both TAs.

BTW: If you want to limit soft phone use to one phone,
I suggest that vPanel have a screen to enter the MAC ID of the Computer/TA being used.
Such a limit may simplify user issues by limiting users from going crazy with phones.