Dialing any digit on an analog phone receives a busy signal

I have a TDM400P with four FXS ports. The analog phone (plugged into port 1 aka Zap/25) has a dialtone and can accept calls just fine.

I receive a busy signal following the first digit I press.

This is the CLI output:
– Starting simple switch on ‘Zap/25-1’
– Hungup 'Zap/25-1’

The Starting simple switch occurs when I pick the phone up and have a dial tone. The Hungup is received after pressing any digit on the phone.

I probably have something simple to change to resolve the issue but I cannot find anything even remotely similar in my searching.

Thanks in advance for any ideas!