Hi Saurabh
If this is happening in one system and not the others (assume all are on the same release/support pack), I would look at configuration in the system for phone number format. Possibly compare the two. Is there an IMG configuration for Phone Number formats (like Addresses and Titles) that you can look at? Perhaps a function person made changes recently (another assumption that it was working fine and has stopped now)
Regards
Colleen