County court claim warning letter

Today I received a letter regarding an unpaid phone bill from a contract I had entered into in June 2028 and says it was assigned to this solicitor in November 2022 for just over £800.

There was a dispute at the time as I had no working service and instead of handling it well I returned the handset and cancelled my direct debit. Being older and wiser I would like to resolve it.

Am I obliged to make this payment? I heard some conversation about there being a time they should have aimed to retrieve it. If yes do I call them and create a payment plan?

Any advice would be great