Yesterday HMRC announced that many of the phone helplines would be closed from 8 April until October, and then dealing only with priority cases. However I see that less than 24 hours later that decision was reversed.
Wait times on the helplines are awful and the only tip I can give you is to ring first thing at 8am, make sure you've already been to the bathroom and have a nice thick book on hand to read while you're on hold 
You have to complete a tax return if HMRC have issued you with a notice to file one or if you have income or gains that need to be taxed - in which case you need to notify HMRC of that fact. If you are due a tax refund, filing a return may be the simplest & quickest way of receiving it.
I have not heard that there is an extension to the filing deadline for tax returns - can't see a reason why that is needed. If you do need/want to complete a tax return for the tax year ending in 5 April 2024, you have until 31 January 2025 to do so (or 3 months after HMRC issue a notice to file, if later).
I can recommend setting yourself up with a Personal Tax Account if you haven't already done so. It's fairly easy to use and covers other benefits too.
HTH
