When you build a lead generation list via an efficient contact form, you can very quickly end up with a long list of contacts to call back. Even if it sounds tempting, never leave a voicemail.
Ideally you should try to call your prospects in the first 5 minutes following their registration on your website. The more you'll wait the lower your chances will be to easily get them on the phone. Some of them might even "forget" that they requested a callback.
But if for some reason you can't set up a quick call after the initial signup, don't be lazy: never leave a voicemail. Most of your contacts will never call back, either because of the low quality / bad connection of your message or because your voicemail will land on their phone at the wrong moment.
If you call them at 10am, when they're at work with their personal phone switched off, and they check their voicemails at 6PM, you'll be the first one of a long list and the chances of getting a call back will be very slim.
Always try to engage in a live conversation with your prospect, even if it requires on average 6 or 7 attempts just to get in touch! I advise you to create stages for each attempt in your CRM (I have clients using Pipedrive and Active Campaign), for instance with a one-day expiry date.
That way, you'll have a clear view on your to-do list and the progress of your attempts. If you manage to connect with your prospect, drag the card to another stage (contact ok) and move on, hopefully to the success stage.