I've been wondering about that plan. I tried the T-mobile $30/mo for limited minutes, SMS and 5Gb internet but SIP was blocked. I could get TLS connected but, being an utter asterisk noob at the time, getting SIP/TLS up at my home end was beyond me (probably still is).
So, looking at this plan (Straight Talk?), they may mean "web" as in "ports associated with browsing" rather than "everything". If so, please let us (or at least me) know. SIP/TLS should work (it's over 443) and IAX2 should work (may have to move port to a commonly open UDP like DNS).
Also, if it's Straight Talk, if there's no T-Mobile connectivity, your voice connection will fail over to other GSM providers America Movil has a deal with (ATT, some regionals, even some GSM towers run by Verizon) - but just voice, not data (not even edge); not sure about SMS.