The Plusnet Hub One has SIP ALG enabled and there is currently no way to disable the SIP ALG feature.
Although the service will generally work if you only have one or two handsets, but the Plusnet Hub One does expose your handsets to the internet so you will experience random phone calls, possibly at inconvenient times. This will be due to hackers scanning your network.
We suggest purchasing a more suitable router for business use, such as a DrayTek router.