You need to make a sip trace using the command
sip set debug on and verify who sent the BYE request. the information is on the from header of the BYE request
example ;
101 HANGUP the call
BYE sip:13052362323@45.32.165.238:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.13:62621;branch=z9hG4bK-524287-1---6950a575d696bb47
Max-Forwards: 70
Contact: <sip:101@192.168.1.13:62621;transport=UDP>
To: <sip:13052362323@45.32.165.238;transport=UDP>;tag=as467448be
From: <sip:101@45.32.165.238;transport=UDP>;tag=607b0105
Call-ID: fxEm4NiMlkSErQdd0rOn2A..
CSeq: 3 BYE
User-Agent: Z 3.15.40006 rv2.8.20
Authorization: Digest username="101",realm="asterisk",nonce="45b13fc8",uri="sip:13052362323@45.32.165.238:5060",response="85b74df16daba2cdf0ea9ffc0501bfcc",algorithm=MD5
Content-Length: 0
13052362323 HANGUP the call
<--- SIP read from UDP:186.149.86.232:61001 --->
BYE sip:101@45.32.165.238:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.15:5060;branch=z9hG4bK329816069
From: <sip:13052362323@186.149.86.232:61001>;tag=237119318
To: <sip:101@45.32.165.238>;tag=as4934dafd
Call-ID:
014e88936a632b9c266216e303f31ac7@45.32.165.238:5060CSeq: 103 BYE
Contact: <sip:130523623230@192.168.1.15:5060>
Max-Forwards: 30
User-Agent: dble
Content-Length: 0