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A poor physical cable between the PBXtra and the carrier can be the culprit of static on a PRI line. A quick method of testing this out is to replace the T1 cable between the PBXtra and the smart jack. You may need a straight-through (standard cat-5) cable, or you may need a http://help.fonality.com/index.php/T1_crossover cable_HOWTO .
Continued testing may also prove out an issue on the cabling down the line to the carrier. Only your carrier will be able to verify if this is the case.
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V29 of the A104DM firmware can cause static and dropped calls, upgrade to V31.
Reference Articles
Configuration Files
/etc/wanpipe/wanpipe#.conf on the PBX server
/etc/asterisk/zapata.conf on the PBX server
/etc/asterisk/zaptel.conf on the PBX server
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