Maximum BLF subscription issue

I wonder if there is a limit on the maximum number of BLF subscription per device for Asterisk 1.2 or Aastra 55i.

When I tried to set up an Aastra 55i with 3 Aastra 536 Expansion modules with about 80 BLF buttons, only the first 50 button would work. “sip show subscriptions” would show only the first 50 BLF subscriptions. Also, when I turned on the SIP Debug on asterisk, I can only see the first 50 SIP SUBSCRIBE request received by Asterisk. If anyone can shred some lights on the problem, it would be great!