There is a known bug in asterisk that causes similar problems. However a message trace is necessary to confirm if this is the source of your problem. The bug is unusual in that some phones will work and others will not under the same conditions but the bug is not in the phone but rather in asterisk. I will not go into an explanation here as it is rather long and it is well documented elsewhere.
Refer to the following --
In sipbroker forum see Another misrouted ACK
In bugs.digium.com see 0011230, 0011326 and 0011545
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