I have developed an online web application to reserve seats using Netbeans. Now I need to add the feature where in when the user calls a particular phone number listed, he would be able to book the seat through phone by entering information like date, seat,etc and making the selection by using the keypad of the phone.
So far I am thinking of using VoiceXML with Tellme voice gateway. I wanted to know if this is the right direction or is there any other technology to implement this feature.
I think you are asking the wrong question here. VoiceXML is a valid technology, not my favorite, but again, a valid one. You can use Twilio, Tropo, TringMe, Nexmo and a multitude of other platforms to create your application.
In addition, you can use Asterisk/FreeSwitch to obtain the same solution, without the need to go into VoiceXML. For example, if you are fluent in JAVA, creating an Asterisk-JAVA AGI program to do the work should be a snap.
I suggest that you take a look at the following links, which can provide some insightful information:
https://blogs.reucon.com/asterisk-java/
These should provide you with ample information in regards to the implementation path with Asterisk.