SIP helper alters SIP traffic when traversing NAT, in an effort to better handle NAT on RTP (=audio) setup.
In most situations, this only makes things worse, get rid of it.
Your phones registers to VOIP provider, every 30 seconds the phone sends a keep-alive or re-register.
These packets are used in ER to keep a connection open (both in NAT table and WAN_IN firewall).
This makes incoming calls work without port map