Introduction When a company is using all the features of VoIP mms business phone system, the phone systems ensure that all the enquires, queries, feedbacks, reviews and complaints of the customers are addressed and customized in the respective departments but the need for in-department calling and department to department calling of the working members within the company comes into play. Memorizing phone numbers, whether personal or professional, of the working members and by the working members within the company is a hectic and time-consuming job. So, here, VoIP extensions or commonly known as extension dialing, offer in-department calling and calling between different departments. What extensions are used? Generally, 4 digit extensions are used by the working members of the company to communicate with each other. Example, 1-2-3-4. Nowadays, bigger companies with large capital and a large number of employees have opted for 3 digit, example 1-2-3, as well as 5 digit extensions, example 1-2-3-4-5. These extensions are similar for the same department, making in-department calls even easier. Different departments have different extensions but they are similar to the main company business extension number. Features 1. Irrespective of geographical location: These extensions can be used irrespective of the location. Calls can be transferred to the different offices open in different parts of the country. Calls can be transferred irrespective of the location of the working members of the company. Calling via extensions is possible even if one employee is sitting in the cabin right next to the other one or the other one is in a different office in some different part of the company. This phone number extension system helped businesses work at full efficiency during the Covid-19 pandemic work from home. 2. Multiple devices: Let us understand this with the help of an example. Let us say an employee has a business handset, personal phone and a laptop. If he gets a call from another employee and if he has enabled and configured them all with VoIP extension, he can have a business handset, personal phone and laptop ringing altogether and can also have them ring in the order of priority, say, business handset followed by laptop and personal phone. Missed call system due to lack of availability is bypassed in these cases. 3. Time-saving: This kills the need of memorizing phone numbers of the working members of the company by the working members of the company. Calling in-between the same department and calling in different departments can be done quickly within seconds. 4. Cost-saving: It reduces the entire cost of in-department calling and calling in between different departments of the company. Managing in-department directories too have additional costs. Conclusion It can be concluded that VoIP extensions save time, money and increase the efficiency of communication within the company irrespective of the geographical location. VoIP extensions, generally, are included in the subscription plan of the VoIP business phone system, either billed annually or monthly. Along with Virtual extension, which generally means external communication with the company’s interested people (customers, investors and suppliers etc), VoIP extensions work internally and virtual extension work externally to give the best communication efficiency of the company at a low cost and small-time period.