Yes, the T1 connecting to the TA624 has to be configured in the DIAL PLAN as well. It will need to be a USER TERM entry. Each T1 is independent, so having only the T1 to the Cisco configured is only half of what you need to do in the ATLAS.
If the TA 624 FXS ports are configured for LOOP START, the the ATLAS T1 should also be configured for Loop Start. With the ATLAS T1 configured, the TA624 FXS ports will get dial-tone from the ATLAS. You can break up the ATLAS T1 by channel if you want each FXS port on the TA624 to have a unique phone number. Otherwise you can configure all 24 DS0s in one entry.
Hope this helps,
Thanks! I'll give this a go sometime soon and mark your answer as correct if it all works for me.
Much appreciated for the assistance!