[Blink] How to force a specific Contact string to be used by Blink

[Tom Wieczorek]

Hi to everyone,

I have a specific problem with one of my SIP providers: The provider 
enforces that the Contact field in the SIP packet corresponds to my 
phone number.

When trying to REGISTER with the provider, Blink uses something like this:

Contact: 
<sip:somerandomnumber at localip:port>;+sip.instance="<urn:uuid:someuuid>"

The provider then replies with:

SIP/2.0 403 Contact User und Anrufernummer verschieden

which translates to something like "Contact User and caller phone number 
different".

Is there any way to force Blink to use the phone number instead of the 
<somerandomnumber> in the Contact field?

Any advice much appreciated.
-Tom