Does it have the issue if an extension to extension call is made, and the Tiptel hangs up while the other extension remains off hook?
Does this happen only on calls over one trunk group? Is there a second group[ to test on?
Is this the only set of this make/model you are using? If not, are others having the same issue?
Have you attempted to swap out the set itself, if so, does a second set have the same issue? If moved provisioned as another extension, does the problem follow the set?
Is this a single line or multi line set? If more than one extension, on the set, do all lines have the same problem?