Overlapping dialling (also known as overlapping requests) is a mechanism integrated in all phones, not just IP phones. It mostly happens when a user dials a number too fast and so the phone is always trying to catch up on itself. This is mostly due to the way the dial plan is configured.
If you are unsure about dial plans, head to this link for more information:
The request part of this question is where the dial plan you have configured is trying to do too many requests at once. For example:
Lets take this American style dial plan and break it down.
1xxx = the international code for America plus any 3 numbers after.
[2-9xxxxxxS0|xxxxxxxxxxxx] - any range of numbers after between 2 and 9 for up to 6 digits and then immediately send OR look for a 12 digit number.
The S0 supersedes the 12 digit part of this plan and means that it will dial out the 6 digits before looking for the 12 digit string.
So for overlapping requests to take effect with the above string, it would be because your phone has immediately dialled the American part of the dial plan and didn't give the phone a chance to look for the 12 digit part of the dial plan. In other words, if you have a 12 digit number you would like to send that begins with 1 and you intend to dial the whole number which is longer than 10 digits, the overlapping requests will appear. This can be prevented by changing the dial plan to a simpler version of a dial plan, which will reduce this issue.
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