Here are some ideas to help you out. You can change the ring tones on your phone's line extensions. Press your phone's Setup button and select menu item 4 (Ring Tone). The rest is dependent on the configuration option you've selected. Some of your options include:
Your phone will always have a private extenstion (e.g. 100). You can use that for internal calls only if you choose. You can configure this button to have a distinct ring tone for internal calls.
For your external calls, one best practice if you are In PBX or Blended mode, is to add an additional extension to each phone in addition to the primary extension (e.g. use additional extension 200 for the phone with primary extension 100). Then route your incoming external calls to the hunt group containing those additional extensions. It can use a different ring tone than your private extension which you use for internal calls. Check out an example use case of this approach on this community at the following link.
Alternatively in PBX or Blended Mode, you can configure shared extensions for your external calls.
If you are in Key System or Blended Mode and have FXO lines, you can use shared FXO lines for your external calls.
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