You need to have a 'No Nine' outdial rule on the Switchvox, specific to your area code, if desired, as in 'Call begins with 615' (for Nashville), and the remaining digits are between 7 and 7 in length...route the call through-- whatever trunk group you use...rule can be final if desired, and allow all users to use this rule, if desired.
Then if that in itself does not work, there is also a digitmap in the Polycom phones with patterns that specify certain call rules (e.g., 9xxxxxxx would be for a 7-digit number prefaced by the digit 9) and there may be one in there for 'internal' extensions that looks something like [1-8]xx (since you said you had 3-digit extension numbers).
The key there is, as long as your local area code doesn't begin with 9, you can modify that Polycom digitmap pattern to be [1-8]xx.T (specifying a number beginning with any digit 1~8, followed by at least 2 digits, then the .T means any more digits to follow, until timeout)
The timeouts are set beneath the digitmap on the Polycom phones, where there's one timeout per pattern, separated by the vertical pipe marks | So the relative pattern for [1-8]xx.T will need a timeout of perhaps |3| (usually) so an internal dialed extension only has to wait 3 seconds until it's transported through the digitmap to the Switchvox's dial rules, whereas a 'No Nine' dialout call (beginning with some area code between 1~8xx (really 2~8xx since there are no area codes that start with 1) and 'any remaining digits' of the 10-digit callback number, until it is transported via the timeout of 3 seconds. That will mean that you've only got 3 seconds to pause when dialing the number, so not much time to pause and verify the remaining digits....you gotta have your tradeoffs..
In this way, the digit pattern transfers from the Polycom phone, when performing a callback of a 10-digit number, over to the Switchvox, where it matches the 'No Nine' outdial rule, and will pass to your outbound trunk group de jour.
Given that you have control of the Polycom phone digitmap and timeout, and the Switchvox Outbound Call Rules, this should work.