Infringing trademark

I have some issues regarding trademarks. I had launched an Android App in the Google Play Store which has a name say AbcdEf. The app is a product itself and helps the user to extend its phone functionality is an easy manner. My business model is B2C even though I deal with business such as Play Stores etc only to distribute my app. The App is available throughout the world even though I live and operate from India. Recently, a very reputed legal firm on behalf of its client which again is a big company sent me a letter stating that I am infringing their trademarks. The company is a big company which has various products one of which whose name is similar to my app's name. The name of one of their product is say ABCDef. The product has a business model of B2B. The company provides services to other organizations & they provide the employees of the organization the ability to use those service through its app which is also there in the Google Play Store. Another way to use the service is IOS App and online. They have the US and international Trademark for ABCDef registered. The letter claims that since their client is very reputed hence the users might mistakenly misunderstand me and my product to be affiliated with them which will affect their reputation. They have provided me a number to call and discuss further. Few differences between them and me are as below 1) Their product category and my product category is totally different 2) The product is totally different 3) They have B2B clients who provide its employees to use their app whereas I have B2C clients. 4) The writing style of the name(trademark) is totally different 5) Target different kind of consumers And few similarities are as below 1) The pronunciation of the trademark is same 2) The product name is similar is even though the capitalization of the letter is different 3) Both the app are present in the Play Store 4) Both would be obviously having Android users Any advice on how I should go about and Am I really infringing their trademark would be really helpful.