It seems very odd that googling 'review opton' (and similar search strings) doesn't find a single review for them (either good or bad).
Further, while I've no idea about the quality of their phone service, they clearly need someone to work on their website:
http://optontelecom.co.uk/
The 'pointing fingers' look like links but they're not. The list under 'Features' look like links but, again, they're not. The bottom of the page is also littered with apparent (but non-existent) links. Some things which shouldn't be links (e.g. their phone number) try to be. While there's a link telling you about cookies, there's no actual warning that the site uses cookies (as required by law).
The company definitely exists. It's been in business since 2010, with its registered office being at an accountant's firm in Acton.
Given that the company uses a 'voip' phone number, it suggests that they route phone traffic across the internet, in a similar way to Skype.