USBI - Mainstreet Telephone Co. offers various communication services, and accessing voicemail through email can depend on the specific features associated with your account. Many modern telephone services provide options to receive voicemail messages as audio files sent to your email. This feature is typically referred to as voicemail to email or unified messaging.
To find out if this service is available for your particular plan and how to set it up, it is advisable to check the current web page of USBI - Mainstreet Telephone Co. They often have detailed information and instructions regarding voicemail services and their integration with email. If this feature is available for you, it can be a convenient way to manage your voicemails.