Either contact an agent or contact the email address given, and there will be a response within five days.
That sounds simple enough, but let's consider what is not also said.
I would venture to suggest that people using this will tend to assume or at least hope that if they receive a message and reply to it, then even if it has taken 5 days for the first response, subsequent responses should be faster.
Also, if the complaint says please review an agent case and gives a reference, the person making the complaint is entitled to hope that the handler will do so.
Thus, a handler response saying please give a phone number and a convenient time to call might be a bit insensitive when part of the problem is the SIM is not working.
If each subsequent complaint handler response is to take a week or more, then if 3 exchanges of messages are needed, that would come to almost a month. If the handler wants to start an argument based on the person not giving a phone number, is that reasonable? How many people whose mobile service is not working would be willing to sit by a landline for a week on the off chance of receiving a call from someone who seems to take typically a week to respond? Alternatively, would the complainant give a number where if they happened not to be in then a message could be left, and if so what would the message from giffgaff be, we called but you weren't there, so these delays are your fault?
I suggest that isn't good enough, and giffgaff needs to review and improve the way it deals with things like this.