Database
[{"_id":1,"MobileNumber":"961xxxxx","Name":"John"},{"_id":2,"MobileNumber":"961xxxx","Name":"Alex"},{"_id":3,"MobileNumber":"971xxxx","Name":"Fatima"},{"_id":4,"MobileNumber":"971xxxx","Name":"Mughees"},{"_id":5,"MobileNumber":"901xxxx","Name":"Mike"},{"_id":6,"MobileNumber":"911xxxx","Name":"Thomas"}]
Query
db.collection.find({MobileNumber:{$regex:"^961"},"$expr":{"$ne":[{"$strLenCP":"$MobileNumber"},10]}})