BLF with pjsip

i have made an install of freepbx 12.0.13.
I have configured blf buttons on aastra 6865i and 6867i to monitor presence.
It works fine but sometimes (about every 5 days), blf don’t work anymore and i must rerstart pabx to make them work again.
Got this error log everytime it happens:
WARNING[13682]: res_pjsip_exten_state.c:337 new_subscribe: Extension 151 does not exist or has no associated hint
It begins with one extension and then grows and grows with other extensions.

Can someone help me?

Thank you very much.