Unless one of the exceptions under rule 33 applies, you must avoid speaking to them about the matter or even listening to what they have to say. The consent exception requires the consent of the other solicitor rather than their client, so you cannot assume from the other party contacting you that you have permission to speak to them, or that they are waiving the benefit of the rule - they cannot do so.
If contact is unavoidable, for instance if a phone call is put through to you or they approach you in person, then as soon as you realise who they are, you should immediately tell them that you cannot speak to them about the matter or listen to anything they have to say, and that any communication has to be through their own solicitor, and you should immediately terminate the conversation.
You should immediately advise the other solicitor of the attempted communication and ask them to advise their client accordingly.