Excerpt of extensions.conf:
[outbound]
exten => 421,hint,SIP/421
exten => 422,hint,SIP/422
exten => 423,hint,SIP/423
exten => 424,hint,SIP/424
exten => 425,hint,SIP/425
exten => 426,hint,SIP/426
exten => 901,hint,park:901@parkedcalls
exten => 902,hint,park:902@parkedcalls
exten => 903,hint,park:903@parkedcalls
exten => 904,hint,park:904@parkedcalls
exten => 905,hint,park:905@parkedcalls
exten => 906,hint,park:906@parkedcalls
exten => 907,hint,park:907@parkedcalls
exten => 908,hint,park:908@parkedcalls
exten => 909,hint,park:909@parkedcalls

Log of 'show hints':
    -= Registered Asterisk Dial Plan Hints =-
                    909@outbound            : park:909@parkedcalls  State:Idle            Watchers  0
                    908@outbound            : park:908@parkedcalls  State:Idle            Watchers  0
                    907@outbound            : park:907@parkedcalls  State:Idle            Watchers  0
                    906@outbound            : park:906@parkedcalls  State:Idle            Watchers  0
                    905@outbound            : park:905@parkedcalls  State:Idle            Watchers  0
                    904@outbound            : park:904@parkedcalls  State:Idle            Watchers  0
                    903@outbound            : park:903@parkedcalls  State:Idle            Watchers  0
                    902@outbound            : park:902@parkedcalls  State:Idle            Watchers  0
                    901@outbound            : park:901@parkedcalls  State:Idle            Watchers  0
                    426@outbound            : SIP/426               State:Unavailable     Watchers  0
                    425@outbound            : SIP/425               State:Idle            Watchers  0
                    424@outbound            : SIP/424               State:Idle            Watchers  0
                    423@outbound            : SIP/423               State:Idle            Watchers  0
                    422@outbound            : SIP/422               State:Idle            Watchers  0
                    421@outbound            : SIP/421               State:Idle            Watchers  0

Log of what's happening:

[Oct 23 18:13:43] VERBOSE[24283] logger.c:
<--- SIP read from 192.168.138.196:5060 --->
SUBSCRIBE sip:901@192.168.138.101:5060 SIP/2.0^M
Via: SIP/2.0/UDP 192.168.138.196;branch=z9hG4bK24311da03^M
Max-Forwards: 70^M
Content-Length: 0^M
To: sip:901@192.168.138.101:5060^M
From: 421 <sip:421@192.168.138.101:5060>;tag=92fff7d8c8243eb^M
Call-ID: 544bc931120cdf82e25f6e18205472f6@192.168.138.196^M
CSeq: 1403371426 SUBSCRIBE^M
Supported: timer^M
Expires: 3600^M
Event: dialog^M
Accept: application/dialog-info+xml^M
Contact: 421 <sip:421@192.168.138.196;transport=udp>^M
Supported: replaces^M
User-Agent: Aastra 9133i/1.4.2.1081 Brcm Callctrl/1.5.1.0 MxSF/v3.2.8.45^M
^M

<------------->
[Oct 23 18:13:43] VERBOSE[24283] logger.c: --- (15 headers 0 lines) ---
[Oct 23 18:13:43] VERBOSE[24283] logger.c: Creating new subscription
[Oct 23 18:13:43] VERBOSE[24283] logger.c: Sending to 192.168.138.196 : 5060 (no NAT)
[Oct 23 18:13:43] VERBOSE[24283] logger.c: Found peer '421'
[Oct 23 18:13:43] VERBOSE[24283] logger.c: Looking for 901 in outbound (domain 192.168.138.101)
[Oct 23 18:13:43] VERBOSE[24283] logger.c:
<--- Transmitting (no NAT) to 192.168.138.196:5060 --->
SIP/2.0 404 Not Found^M
Via: SIP/2.0/UDP 192.168.138.196;branch=z9hG4bK24311da03;received=192.168.138.196^M
From: 421 <sip:421@192.168.138.101:5060>;tag=92fff7d8c8243eb^M
To: sip:901@192.168.138.101:5060;tag=as189c60c2^M
Call-ID: 544bc931120cdf82e25f6e18205472f6@192.168.138.196^M
CSeq: 1403371426 SUBSCRIBE^M
User-Agent: Asterisk PBX^M
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY^M
Supported: replaces^M
Content-Length: 0^M
^M

<------------>
[Oct 23 18:13:43] VERBOSE[24283] logger.c: Really destroying SIP dialog '544bc931120cdf82e25f6e18205472f6@192.168.138.196' Method: SUBSCRIBE