Simply speak into your phone or computer while recording a greeting with your business phone service. If you need to create your own audio file, the voice memo app on your phone should do fine.
As a professional business, in no way should you ever resort to leaving one of those generic, pre-recorded, “No one is here to take your call” messages that the phone company often provides. Record a greeting yourself, or have one professionally done for you. If you choose to record a greeting yourself for your office phone, there are a few simple keys that you should keep in mind:

Let’s be honest, you (hopefully) set up your voicemail when you first got your phone, and it probably hasn’t changed since then. If you’re about to start job-hunting, now is the perfect time to refresh your professional voicemail greeting.
Remember to watch out for ambient noise on your end of the call as well. Train signals, traffic, or even outside conversations can be very distracting to your callers and project an unprofessional image. Don’t forget the beep. Every millisecond – yes, millisecond – between the end of your greeting and the beep is time callers could potentially drop the call. That’s why the anticipated beep should sound the moment your greeting is finished. Leaving lengthy, silent gaps may cause your customers to wonder if they’ve been disconnected. Don’t give them the opportunity to hang up waiting to hear that affirmative tone. Check and then recheck your greeting. After you complete your outgoing voicemail message, play it back to verify you’ve said everything you intended to say. Then take it a step further and dial in to experience the greeting for yourself, and be mindful to contemplate the greeting from the customer’s perspective.