Teodor, nobody said that SIP uses port 1720.
To the contrary, the person that initiated the posting is trying to use SIP BECAUSE port 1720 is being blocked by the provider.
My point is that is port 1720 (used by H323) is being blocked, most likely port 5060 (used by SIP) is also blocked.
Better just to use other port and keep the H323 gear that they already have.
Best regards.
Wilson Boyrie.