If an internet modem is being used to access Telstar such as the a WiModem232 (https://www.cbmstuff.com/proddetail.php?prod=WiModem232OLED) there are a few things to be aware of.
Videotex terminal software does not normally send a CR (0x0D) but uses the ASCII underscore character (0x5F) instead. The videotex character set is slightly different to ASCII and the code 0x5F represents a hash/pound (#) when using the videotext character set. This can make it difficult to configure a modem using this software unless the modem can be configured to accept 0x5F (#) as a CR.
The WiModem232 has a special Telstar mode that can be enabled/disabled using the AT*WS command which, when enabled, supports the 0x5F (#) character as a CR. This means that the modem can be configured easily with videotex terminal software in the same way as normal terminal software. The commands are detailed below.
The Arduino based modem described in the article Building an Arduino Based TELSTAR Modem automatically supports the 0x5F (#) character as a CR.
Some software can send CR using a control codes. For example using Ctrl M with Commstar will send a CR. Using the Antelope software, Ctrl D can be used. However please see the section below.
Videotext systems tend to transfer data using 7 bits with the eighth bit being used for parity (7E1), as a result some videotext terminal software does not have the ability to change this. This can make it difficult to configure a modem using this software unless the modem can be configured to ignore parity. The WiModem232, when set to TELSTAR mode using the AT*TS command (described above) will ignore parity to allow AT commands to be used successfully.
The Arduino based modem described in the article Building an Arduino Based TELSTAR Modem always ignores parity.