Great suggestion. since in Pakistan , we have only port 5060 blocked on Tandum Level . Therefore, changing the SIP port , other than standard port can be a way around. However, some of the ISPs filters on DPI and can be identified even if we use SIP on any other port. In this case, as mentioned above, SIP over TLS is a best match.
SIP over TLS hide your signaling but there is always a chance of RTP traffic being monitored in some cases. If you are identified as carrying RTP stream over your network, there is a chance of blacklisting of your IP.
VPN is a work around for both cases. But the introduction of delays and high bandwidth consumption is a "not so good" ! Specially when you have bulk of traffic over your network.