What do you mean, Dual Mode ? 2G+3G ? Or 2G CS+PS ?
You need to ensure your UE are :
1/ doing periodic location update (T3212 = 4hrs for example)
2/ autonomously attach to the network when switched on (parameter “ATT” = ON)
3/ that the periodicity of location update (timer = T3212) on BSS side is shorter than periodicity of IMSI detach in VLR (timer = purge or detach timer)
Those are the basic rules. If they are done, but still no success, then deeper investigations must be performed. The interesting traces to do are @ A interface and/or IuCS level. Is the paging sent to the idle UE ? To which LAC specifically ?
When turning your MS on, does it perform an Attach, through a location update.
After few hours in idle mode, does it send a location update (periodic) ?
After few hours in idle, when trying to page this MS, does the MS receives the paging on the radio ?Does it try to reply ?
In BSS Counters, check out the follwoing counters, in an cell with such issue :
SDCCH established for Paging Response
SDCCH established for Location Update
SDCCH established for ATTACH or DETACH
Number of Pagings sent
Then compare those numbers with a “good” cell, if any. Are the ratio approximatively similar ?