Brainstorming on a possible PBX/PRI problem and could use some help...

Hey everyone! I'm not really looking for solutions on this, I just want to make sure my troubleshooting....thought process...is correct.

So we have an old Toshiba CTX100 at one of our remote sites. The phone system has been having significant problems as of lately and I am completely out of ideas as what else to try. I am not a VOIP person at all BTW. The phones in the office can find the PBX and connect to it without a problem. They are on their own VLAN with QoS. The network is very simple... two VLANs: voice and data... Gigabit ethernet Cisco 2960 switch with PoE. Switch is working fine. No changes have been made to the network, or the PBX at all.

Here is the issue...

• Phone calls are randomly dropped. They are not disconnected, just audio drops.
• Also, when you hang up on the outside, the phone does not terminate the call (not a big deal)

Here is what I know so far...

• Can ping PBX
• Can ping Phone
• PBX sees phone
• Phone sees PBX
• Phone and PBX both see VM server on a separate subnet/VLAN (I know that ACL's are fine.)
• Phone has dial tone, can dial out local and limited long distance
• Can dial from outside to phone with no issue.
• I was able to call in from my cell and maintain a connection for at least 20 mintues. (I had a timer and just let it keep going while I was reading about the PBX).
• I was also able to do the opposite, and maintained a call for the same amount of time.
• I did experience random dropouts though, but I do not think they are the providers fault
• We have a PRI line, but like I said I think the line is fine.
• We have a school next door with the exact same setup and no issues, with the same provider (though not on same phone circuit, same WAN circuit).
• I checked the cabling and it was fine
• I checked for known service outages and there were none.

My suspicion is that the line cards in the PBX are failing. It is an older system... I think 10+ years old, so we are not to crazy about supporting it. The office will be vacant in a month so we are trying to build a case to forget about it.

Is there anything I am missing though? I hate VOIP with a passion, unless it is Alworx, AVAYA, or Cisco. I can handle those...