m0n0wall Forum - Port mapping with pub IP bridged wan-opt for asterisk

I have m0n0wall working on public IP and have multiple public IPs. I have setup optional interface bridged with wan and it's working fine with my behind the monowall asterisk.

Now issue is that some of my customes dont' want to use standard sip 5060 port and they want to ahve some thing like 5098 due to some resitcitions at thier end. How i can resolve this? I don't want to put the asterisk on NAT with private IP if this is not the only option.

Waiting for response. thanks in advance.