Here is the problem scenario.
The IPT installation was running fine with 7960 and 7920 and ATA devices installed to a Callmanager Express 4.1. Later we needed more wireless phones, so ordered the new 7921 phones as 7920 phones are no loger in market. In order to make the 7921 phones work we upgraded the IOS in 2851 router from 12.3(11)T8 to 12.4(15)T1.
The 7921 phones are now working fine, but we have trouble with the 7920 phones. Everytime there is MWI, an error message pops up on the screen " External call unknown". To get rid of the message we need to remove the battery and reset the phones. Next time MWI appears, again the same error message pops up.
I am attaching the picture of the error message herewith. Any help would be appreciated.