When you get a busy signal, dial *66 to use Busy Redial and you will get a call back when the line you are calling is available. Busy Redial is available only for telephone numbers within the continental United States.
Advanced greetings provide additional functionality for power users, but require additional setup. Contact the UBIT Help Center if you are interested in any of these options and require assistance:

If it works better for you, offer your callers the option to email you, or even send over a text to a VoIP texting app, or even your personal number if you don’t mind. Many people will prefer the ability to quickly text or email, and callers will appreciate the increased level of service.
To get voicemail messages to their intended recipients faster, many speech-to-text providers now offer voicemail transcription services. Nuance is one such company, and Dan Faulkner, its senior vice president of mobile, says the service is widely popular. "We find that the voicemail-to-text service is incredibly sticky," he said in a recent interview with Speech Technology magazine. "When people get it, they don't listen to voicemails anymore."