Tap “Call Settings,” and then tap “Voicemail Settings.” Tap “Voicemail Number.” Type the voicemail number into the box or edit the existing voicemail number. Tap “OK” to save your changes. Tap on Call settings and select Voicemail. Tap on Voicemail service and select My carrier or My operator. Tap on Setup, select Voicemail number and type in your voicemail number. Tap on OK in the Voicemail number changed popup.
You can also upload an unavailable greeting using the OnSIP app. After logging in, click on the Voicemail Settings gear icon at the far right. Make sure that the Custom Greeting is toggled to "on." You will then see an option to upload a .wav file of your choice. This recording will serve as your unavailable message.

Website: https://behavioralhealthconnection.net/blog/3-ways-improve-your-therapist-voicemail-script
To choose the best answering machine, you first should become familiar with the different models that are on the market. As with most kinds of technology, new answering machines are developed often. You might find some models that are attached to telephones and other models that are freestanding but are designed only for certain phones. Some answering machines use tapes to record incoming and outgoing messages, but more advanced models tend to use electronic recordings. It is common for fully electronic answering machines to hold more information.