You can try dialing a # once you are done dialing the numbers. From what I understand, your service supports 7, 10, and 11 number dialing, and will wait a requisite amount of time before assuming you have completed entering numbers. (Although I wouldn't expect this with an 11 number entry.) The # effectively tells it that you are done dialing.
At any rate, give that a shot.
That said, I do sometime experience the same issue as you, albeit sporadic.
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