ip phones won't answer on handset

I have several IP phone connected to my Nortel BCM400. A handful of the phones have the following problem:
-When trying to answer calls, picking up the handset does not always answer the phone. Answering on speaker works more frequently.
-When trying to make calls, same type of issue. Picking up the handset you can not always get a dial tone right away.

It feels like the phones have a delay trying to connect to the phone system. Though they do ring just as fast as digital phones when calling them.
I have HP4108gl switches, everything is on the one vlan (not best practice i know) there is a fibre link between switches. one switch has the phone system connection, and the ip phones are on the other switches. Not all phones are effected it appears.
I suspect it is networking related but not sure where to look first.
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You need to first put your phones and phone system into its own vlan and add prioritization to that vlan. Your data going across your network maybe causing the RTP and call control messaging traffic to be delayed or lost.
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