Yes, mlchef, it is unreasonable to expect a TA to be "on call" 24/7! (They have a life, too!) If you've booked with a "large" agency, there should be someone "on call" all the time!
IMHO, a good TA is worth their weight in gold! It may cost a little more in the long run, but a good TA is worth every penny, IMO!