There is currently no way to disable SIP ALG on the Sky Hub.
Although the service will generally work if you only have one or two handsets, you may experience call dropping, issues getting your handsets on-line, and issues such has not hearing your caller.
We suggest purchasing a more suitable router for business use, such as the Technicolor or DrayTek. However, we should point out that Sky broadband is not permitted for business use and may not permit you to use a router other than the Sky Hub.