I am running a 2821 with IOS version 12.4(20)T4.
I set up Meet-Me and it was working initially. I could get several people into a conference. It's been about 3 months since I set it up. Today was the first day the client wanted to use it, and it didn't work. The odd thing is that they were able to get the conference started, a second person could enter the conference, but when a third person tried to enter that person got a busy signal. I did a shut/no shut on the dspfarm. I have not tried restarting the router because it's being used pretty frequently until closing time.
Configure the ephone-dn for the meetme with octo-line
ephone-dn 20 octo-line
or configure dual line with same number 2or 3 as you required.
This will solve your issue