I use a 1950s vintage butt set with a rotary dial for most things. The telco techs usually make fun of it, but I have yet to find anything critical that I have needed to do where it wouldn't function. It's just fine for listening to T-1 circuits and trying to hear resynch issues.
The data lockout stuff will help you not screw up a data circuit if you accidentally come across one with the TALK switch on. Is that important to you?
Don't forget that you will also need a Fox and Hound set. If you work on T-1 circuits, you really need a T-1 BERT at least, and a full protocol analyzer is a nice thing to have.
No reason not to just get a cheap butt set, though.
The Stanley push drill.
scott
"C'est un Nagra. C'est suisse, et tres, tres precis."