This does happen from time to time and the most common cause is an administrational oversight on behalf of your previous telecoms provider.
Firstly, you should check the bill is actually a mistake, by ensuring none of your services remain with your previous provider.
Contact your account managers who may be able to assist or otherwise, get in touch with your previous provider, directly. You should quote your account number and be prepared to answer any security questions.
If your previous provider was O2* and all attempts to resolve the issue have failed, you can call our customer service team on 03300 881818 or write to us via email@example.com, providing full details. We may be able to assist, but cannot guarantee a resolution.
*We don't have an agreement with any provider other than O2 and will be unable to assist if you were not previously an O2 customer, immediately prior to switching to plan.com