You could place a button on your main layout which would activate a script step to create a new window. Part of that script step will allow you to control both the size of the window and its location.
If the messages are in a separate table occurrence, and related to the main layout, you could have the message(s) in a portal instead of a separate window.
We do a similar thing with vendor details for materials. On the main materials page, it simply lists the vendor name and ID# along with material details and stock items/locations/usage. When you want details on the vendor, I use a GoToLayout rather than a new window. The new layout looks a whole lot like the old, but has vendor info instead of the stock info. When the layout switches, it appears as though you've simply swapped vendor info (your phone messages) for stock info (your other data).
For you, put a button next to the phone# that says "see messages" and have that button switch to the other layout showing messages in place of non-pertinent info. Put a button on the new layout to send you back to the original layout.
The idea of a portal is also good, you can put the portal on your new layout to show the individual messages...then flip back to the first layout with a button click. This avoids window resizing, closing the window and resizing when you're done, etc.
Note: you could do the same thing with tabs rather than different layouts if it makes sense to do so.
This is great. Thank you.