You are assuming they don't already have a suitable firewall in place?
There are many ways to provide protection such as limiting the access to specific IPs, using TLS etc.
I don't know what f/w they have in place, but a f/w that can also do SBC functionality is a pretty major piece of gear, and expensive. Wouldn't expect a company with one of those to balk at the very low cost of an MBG with a single teleworker license.
If you use TLS, which is recommended, then your f/w must be a proper back-to-back (B2B) user agent (either sip or minet) in order to manipulate the signalling.. and, generally speaking, they also perform user auth.
Allowing TLS through to the call server isn't going to provide any additional security.