5060 blocked issue

[advant]

First of all, I would like to thank you all for the
amazing wealth of information that is contained
in all these forums.

Here is my issue:

Port 5060 (both incoming and outgoing) is blocked by my ISP. I currently use us.voxalot.com:4060 as my SIP server on my ATA. My ATA listens on 5061 to bypass the incoming port block. ATA registers fine.

However, I cannot make any outgoing or incoming calls with this setup. When I make an outgoing call, it rings the destination phone. Calls do not get completed. The same happens for incoming calls as well. I hear the rings but the calls do not get completed when the phone goes off hook.

I have traced the root of the problem as follows:

While the adapter sends the REGISTER messages to the correct unblocked 4060 port, it responds to INVITE messages to the default 5060 port. This
is probably because the INVITE messages from the Voxalot server contain the IP address only and not the port # specification. The adapter attaches the default port # which is 5060 and sends the ACK messages to a blocked port.
The adapter referred here is a Grandstream HT486.

This is the reason I find calls do not get completed. Is there any way to work around this issue? Can something be done on the server side to correct this issue?

Any help with this issue is very much appreciated.

Thanks in advance.