As this is your first post welcome to the Pure Planet community. When you post here you are not talking directly to Pure Planet but to your fellow customers. They are usually a helpful bunch.
But in this case, if you want to contact Pure Planet, you need to open the app, click on the three little bars top right hand corner, select Help, then choose Wattbot, put 'message the team' in the question box and then explain the problem. They promise to get back to you within one or two working days.
I'm a little surprised you were phoned about this - my understanding is that adjustments to direct debit payments are usually communicated by email. But I could be wrong. Bear in mind that everyone's payments go up by 50% in October because of the summer/winter payments split.
My final comment would be - are you sending in your monthly meter readings? Because if you are, you and Pure Planet would know that your usage has been very low...
Hope you can get it sorted. Post back here if you have any further questions.