Asterisk ignores port in Register directive

I facing this error in both Asterisk 1.2.18 and 1.4.6. I would appreciate any insight.

I have the following register string in sip.conf

register=username:password@192.168.14.250:6060/username

Asterisk should thus send the register request to: 192.168.14.250 at port 6060. However, it actually sends the request to the standard 5060 port. Here is the output from CLI

Retransmitting 0000005 (NAT) to 192.168.14.250:5060:
REGISTER sip:192.168.14.250:6060 SIP/2.0
Via: SIP/2.0/UDP 192.168.14.104:5070;branch=z9hG4bK3a63040b;rport
From: sip:username@192.168.14.250;tag=as7857a350
To: sip:username@192.168.14.250
Call-ID: 7dba5db402361ef80ba686c94f60bc51@127.0.0.1
CSeq: 102 REGISTER
User-Agent: Asterisk PBX
Max-Forwards: 70
Expires: 120
Contact: sip:username@192.168.14.104:5070
Event: registration
Content-Length: 0

Does anyone know how to send the register request to a non standard port (6060)