Best Practices for Extensions


By doing a little advance planning on the extensions in your organization, you can make your users more reachable and fully exploit the functionality of the OnSIP Hosted PBX.

Extension Limitations

Extensions have a max of 6 digits, can start with a 0, but have to be at least two-digits; cannot be one digit.

Extensions should be at least four digits and start with a 7

You will have the best success with extensions that are four digits long and start with a seven. Many customers have extensions beginning with a '1' and would like to keep their extensions the same. The first concern with extensions beginning with a '1' is that they may conflict with your "Attendant" Menu. You may, for example, have the option on your Attendant Menu to: “Choose 1 for sales, 2 for customer service, etc.” Then, if the caller chooses '1,' we have to wait four seconds to see if they actually wanted '1' for sales, or if they hit '1' as part of an extension, '101.'

The second concern is with Polycom phones and other phones that have a digit map or some kind of internal dial-plan. As soon as the caller dials the '1,' the phone expects you to dial 10 more digits with the next digit being a number between 1 and 9. So, as soon as you dial '10' on your way to dialing '101,' the phone will give you a fast-busy.

Extensions that are four digits long give you the flexibility to designate the third digit (the digit in the 'hundred's place) to something specific, e.g. a voicemail box or cell phone.

Extensions for voicemail boxes, cell phones, groups, and ACD queues

In order to send calls directly to a user's voicemail box, that voicemail box must have a direct dial extension. To add an extension to a voicemail box, log into the OnSIP Admin Portal and choose the "Resources" tab. Then, choose "Voice Mailbox" from the 'Type' dropdown to see all the voicemail boxes on the system. Give each voicemail box its own extension.

You can do the same for cell phones. Create a new Resource of type "External Phone Number." Enter the external phone number, be it a cell phone, home phone, or other frequently dialed external number. Save the number. Once it shows up in the list below, click on the number and assign it an extension.

Similarly, groups and ACD queues can be labeled by their own extensions. ACD Queues are found under the "Apps" tab of the Admin Portal. By adding these extensions, you can forward calls to a general queue or group, which would allow any member to answer.


For example, if the extension for the user is 7001, you can create an extension for their voicemail box that is 7701. Do the same for all extensions. Then, if you know someone's extension, just by changing the third digit to a '7' you then know the direct transfer extension for their voicemail box.

You can also do this for cell phones. They could be the '9' extension. Then, when finished, you could have user Sue Smith be extension 7001 for her phone, 7701 for direct transfer to her voicemail, and 7901 to send calls to her cell phone.


Not Recommended

The following extension configuration is not recommended: *711 *711x Extensions starting with these digits will go to the TTY Relay Center; *411 *411x Extensions starting with these digits will go to Information; *611 *611x Extensions starting with these digits will report a problem with telephone service (landline service); *911 *911x Extensions starting with these digits will got to Emergency Call Centers if enabled in customer's account. 988 is reserved for the Suicide Prevention Hotline.


Updated July 2015

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    Jason Wagner

    Good to know! Thank you.