To give a sensible answer we need to know what sort of phone, and also what sort of phone system at your office. And your method of connecting from home to office.
Just speaking about standalone SIP phones, like the Gigaset that I use for our home phone, these actively register with the services they will use. Assuming even half decent firewall features, this does not leave any port open to anything except the service(s) you register with.
If your phone is registering with your office, it really depends on the phone system. For example our Cisco system has facilities for phones and soft phones to register securely over the Internet to the office. Or we can run a VPN and let the phone register directly.
One quick thing you can check, most providers want any form or SIP ALG or inspection disabled on your router. So you could have a look at your Plusnet router, see if that feature exists and make sure it's disabled.
