An Explanation...
What we're up against is technology changes in the Phone Company.
Telephone Service used to be all analog connections and switches - Once a call was connected (answered), there was a physical copper connection, through switches and relays, between the devices. The modems in these terminals were designed in the 80's and expect that same responsiveness and clarity.
Today, calls are translated into digital data, lets say "up trans-coded", as soon as possible, and carried to the trans-coder nearest to the recipient over Ethernet and "down trans-coded" back to analog and sent to the recipient.
To a human ear, an small click or change in the persons voice tone is usually imperceptible, or, even if perceptible, not important.
With the machines, there are checks and balances to ensure that every whistle, click, buzz and chirp arrives exactly the same as it was sent.
With the changes in Telephone Circuitry, also came changes in Error Control Protocols, allowing newer modems to allow for and adapt to the changing technology. Unfortunately, the VeriFone TRANZ terminal Modem cannot incorporate the new protocols.
As a result, we have to make every phone connection using the error control and speeds that were designed into the Modem electronics, specifically, BELL protocol and either 300 or 1200 baud.
Possible Solutions...
Change Clock Communication Speeds
If your TRANZ terminal currently has the modem speed set to 1200 baud (default), try setting it for 300 baud. This is done through the Clock Menu, MODEM SPD item
https://sundialtimesystems.freshdesk.com/support/solutions/articles/1000222062
Lock IN Computer Comunication Speeds
You can provide Explicit Protocol Instructions to the Modem in the Initialization String. These will vary based on the Modem, but are often settings like "AT+MS=B212,0,1200,1200" to signify the Bell 212 Protocol, Disable Speed Negotiation, Use 1200 baud outgoing, Use 1200 baud incoming.