Your network can be the culprit causing the issue for the application. Sometimes, the network can be blocking the application usage for your phone. Several users solved this issue by resetting their network settings. Resetting network settings will renew all the information related to the network that you are using on your phone. You can reset the network settings by following the steps below:
You have the option in settings for calls to be answered either via the app itself (check off iOS Device) or by having the call forwarded to your mobile number. Either way, you are not technically doing the recording on your iPhone. It's all done on Google's servers, which handle the Voice over IP (VoIP) connection.

If you are recording a voicemail message for business, be sure you include your name and your company’s name, so people know who they are calling and were they are reaching them. If you do not include the business name, they may think they have mistakenly called you at your personal number. Step One: Introduction. Start with an upbeat greeting that includes the name of the person you’re calling: “Hi, Tim!” “Hello, Susan!” “Good morning, Tom!” Then, introduce yourself by giving your name, company name if applicable, and telephone number. Step Two: Message. Step Three: Recap and sign off. What is a professional voicemail greeting?
The process is very simple. You'll just have to click the green phone icon, the same place you make calls, and navigate to the voicemail screen. On the voicemail screen, you'll see a greetings option on the top left.