I think yahoo people has implemented a new thing or may be there's a problem with ebuddy...
If this is yahoo's new implementation than:
After collaborating with all major GSM service provider in India.
They have implemented this new feature, that means you need to be subscribed to your GSM provider, if you want to login into yahoo messenger using your mobile device.
If you have not subscribed, you may get such an error:
MSISDN not found. Please contact your Operator.
What to do if you have subscribed and are not able to login due to this error?
Try to contact you service provider or login through: http://www.ebuddy.com (this one of the many online messengers available on net, you can also try out others.)
If its an error caused by ebuddy than:
I have noticed this when using ebuddy that many times when you logout of your account using ebuddy it does not logout and you are left logged in.
May be the error is because of this.
If you have more information regarding this problem than please post some comments.
I hope that this was useful to you.