I was with Primus a while ago back when it was possible to have o2 broadband (which no longer exists) without having an o2 landline so I used Primus for line rental and o2 for broadband (via LLU).
The only problems I had with Primus was less than a months notice was given when they increased their line rental for which they sorted for me.
I also had problems when trying to leave my contract a little early. Ofcom had previously stated that those wishing to leave contract early may do so without paying full line rental charges for any remaining months instead stating it should be a lot less because it no longer costs them once customer moves. I quoted to Primus what Ofcom had stated and provided them a link to Ofcom's website where the likes of other main providers had agreed to reduce their prices accordingly for those wishing to pay off the remaining months and leave early.
Naturally, Primus said that Ofcom guidelines did not apply to them and only applied to the big providers and refused to enter into further correspondence or allow me to leave without paying FULL line rental for the remaining months.
I complained to the ombudsman and provided them with link to Ofcom website where Ofcom had put they expected other providers to follow suite as the big providers have. The ombudsman agreed in my favour and forced primus to reduce their early termination charges accordingly which they reluctlantly did.
What primus did and their responses I expected to be honest as most other landline providers probably would have tried it on as well.