Twinkle
I'm afraid I haven't completed the UC online form so I can't give a detailed answer, perhaps other members can comment.
So, if the field for the phone number is required then I would expect a valid telephone number; landline or mobile will be required, if it is not then it can be left blank.
You can give someonelse's number but it is not going to be of any value in managing the account as the DWP will not talk to anybody who is not the claimant or their appointee.
You can request contact by email, it's up to the DWP whether they allow it but it won't help with the claim process as the request would be completely separate from this.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems